path: root/doc/src/sgml/func/func-datetime.sgml

  <sect1 id="functions-datetime">
   <title>Date/Time Functions and Operators</title>

  <para>
   <xref linkend="functions-datetime-table"/> shows the available
   functions for date/time value processing, with details appearing in
   the following subsections.  <xref
   linkend="operators-datetime-table"/> illustrates the behaviors of
   the basic arithmetic operators (<literal>+</literal>,
   <literal>*</literal>, etc.).  For formatting functions, refer to
   <xref linkend="functions-formatting"/>.  You should be familiar with
   the background information on date/time data types from <xref
   linkend="datatype-datetime"/>.
  </para>

  <para>
   In addition, the usual comparison operators shown in
   <xref linkend="functions-comparison-op-table"/> are available for the
   date/time types.  Dates and timestamps (with or without time zone) are
   all comparable, while times (with or without time zone) and intervals
   can only be compared to other values of the same data type.  When
   comparing a timestamp without time zone to a timestamp with time zone,
   the former value is assumed to be given in the time zone specified by
   the <xref linkend="guc-timezone"/> configuration parameter, and is
   rotated to UTC for comparison to the latter value (which is already
   in UTC internally).  Similarly, a date value is assumed to represent
   midnight in the <varname>TimeZone</varname> zone when comparing it
   to a timestamp.
  </para>

  <para>
   All the functions and operators described below that take <type>time</type> or <type>timestamp</type>
   inputs actually come in two variants: one that takes <type>time with time zone</type> or <type>timestamp
   with time zone</type>, and one that takes <type>time without time zone</type> or <type>timestamp without time zone</type>.
   For brevity, these variants are not shown separately.  Also, the
   <literal>+</literal> and <literal>*</literal> operators come in commutative pairs (for
   example both <type>date</type> <literal>+</literal> <type>integer</type>
   and <type>integer</type> <literal>+</literal> <type>date</type>); we show
   only one of each such pair.
  </para>

    <table id="operators-datetime-table">
     <title>Date/Time Operators</title>

     <tgroup cols="1">
      <thead>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         Operator
        </para>
        <para>
         Description
        </para>
        <para>
         Example(s)
        </para></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>+</literal> <type>integer</type>
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Add a number of days to a date
        </para>
        <para>
         <literal>date '2001-09-28' + 7</literal>
         <returnvalue>2001-10-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>+</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Add an interval to a date
        </para>
        <para>
         <literal>date '2001-09-28' + interval '1 hour'</literal>
         <returnvalue>2001-09-28 01:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>+</literal> <type>time</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Add a time-of-day to a date
        </para>
        <para>
         <literal>date '2001-09-28' + time '03:00'</literal>
         <returnvalue>2001-09-28 03:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>+</literal> <type>interval</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Add intervals
        </para>
        <para>
         <literal>interval '1 day' + interval '1 hour'</literal>
         <returnvalue>1 day 01:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp</type> <literal>+</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Add an interval to a timestamp
        </para>
        <para>
         <literal>timestamp '2001-09-28 01:00' + interval '23 hours'</literal>
         <returnvalue>2001-09-29 00:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time</type> <literal>+</literal> <type>interval</type>
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Add an interval to a time
        </para>
        <para>
         <literal>time '01:00' + interval '3 hours'</literal>
         <returnvalue>04:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <literal>-</literal> <type>interval</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Negate an interval
        </para>
        <para>
         <literal>- interval '23 hours'</literal>
         <returnvalue>-23:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>-</literal> <type>date</type>
         <returnvalue>integer</returnvalue>
        </para>
        <para>
         Subtract dates, producing the number of days elapsed
        </para>
        <para>
         <literal>date '2001-10-01' - date '2001-09-28'</literal>
         <returnvalue>3</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>-</literal> <type>integer</type>
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Subtract a number of days from a date
        </para>
        <para>
         <literal>date '2001-10-01' - 7</literal>
         <returnvalue>2001-09-24</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>date</type> <literal>-</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Subtract an interval from a date
        </para>
        <para>
         <literal>date '2001-09-28' - interval '1 hour'</literal>
         <returnvalue>2001-09-27 23:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time</type> <literal>-</literal> <type>time</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract times
        </para>
        <para>
         <literal>time '05:00' - time '03:00'</literal>
         <returnvalue>02:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time</type> <literal>-</literal> <type>interval</type>
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Subtract an interval from a time
        </para>
        <para>
         <literal>time '05:00' - interval '2 hours'</literal>
         <returnvalue>03:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp</type> <literal>-</literal> <type>interval</type>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Subtract an interval from a timestamp
        </para>
        <para>
         <literal>timestamp '2001-09-28 23:00' - interval '23 hours'</literal>
         <returnvalue>2001-09-28 00:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>-</literal> <type>interval</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract intervals
        </para>
        <para>
         <literal>interval '1 day' - interval '1 hour'</literal>
         <returnvalue>1 day -01:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp</type> <literal>-</literal> <type>timestamp</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract timestamps (converting 24-hour intervals into days,
         similarly to <link
         linkend="function-justify-hours"><function>justify_hours()</function></link>)
        </para>
        <para>
         <literal>timestamp '2001-09-29 03:00' - timestamp '2001-07-27 12:00'</literal>
         <returnvalue>63 days 15:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>*</literal> <type>double precision</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Multiply an interval by a scalar
        </para>
        <para>
         <literal>interval '1 second' * 900</literal>
         <returnvalue>00:15:00</returnvalue>
        </para>
        <para>
         <literal>interval '1 day' * 21</literal>
         <returnvalue>21 days</returnvalue>
        </para>
        <para>
         <literal>interval '1 hour' * 3.5</literal>
         <returnvalue>03:30:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>interval</type> <literal>/</literal> <type>double precision</type>
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Divide an interval by a scalar
        </para>
        <para>
         <literal>interval '1 hour' / 1.5</literal>
         <returnvalue>00:40:00</returnvalue>
        </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <table id="functions-datetime-table">
     <title>Date/Time Functions</title>
     <tgroup cols="1">
      <thead>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         Function
        </para>
        <para>
         Description
        </para>
        <para>
         Example(s)
        </para></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>age</primary>
         </indexterm>
         <function>age</function> ( <type>timestamp</type>, <type>timestamp</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract arguments, producing a <quote>symbolic</quote> result that
         uses years and months, rather than just days
        </para>
        <para>
         <literal>age(timestamp '2001-04-10', timestamp '1957-06-13')</literal>
         <returnvalue>43 years 9 mons 27 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>age</function> ( <type>timestamp</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Subtract argument from <function>current_date</function> (at midnight)
        </para>
        <para>
         <literal>age(timestamp '1957-06-13')</literal>
         <returnvalue>62 years 6 mons 10 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>clock_timestamp</primary>
         </indexterm>
         <function>clock_timestamp</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (changes during statement execution);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>clock_timestamp()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>current_date</primary>
         </indexterm>
         <function>current_date</function>
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Current date; see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_date</literal>
         <returnvalue>2019-12-23</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>current_time</primary>
         </indexterm>
         <function>current_time</function>
         <returnvalue>time with time zone</returnvalue>
        </para>
        <para>
         Current time of day; see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_time</literal>
         <returnvalue>14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>current_time</function> ( <type>integer</type> )
         <returnvalue>time with time zone</returnvalue>
        </para>
        <para>
         Current time of day, with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_time(2)</literal>
         <returnvalue>14:39:53.66-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>current_timestamp</primary>
         </indexterm>
         <function>current_timestamp</function>
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_timestamp</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>current_timestamp</function> ( <type>integer</type> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction), with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>current_timestamp(0)</literal>
         <returnvalue>2019-12-23 14:39:53-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>date_add</primary>
         </indexterm>
         <function>date_add</function> ( <type>timestamp with time zone</type>, <type>interval</type> <optional>, <type>text</type> </optional> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Add an <type>interval</type> to a <type>timestamp with time
         zone</type>, computing times of day and daylight-savings adjustments
         according to the time zone named by the third argument, or the
         current <xref linkend="guc-timezone"/> setting if that is omitted.
         The form with two arguments is equivalent to the <type>timestamp with
         time zone</type> <literal>+</literal> <type>interval</type> operator.
        </para>
        <para>
         <literal>date_add('2021-10-31 00:00:00+02'::timestamptz, '1 day'::interval, 'Europe/Warsaw')</literal>
         <returnvalue>2021-10-31 23:00:00+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>date_bin</function> ( <type>interval</type>, <type>timestamp</type>, <type>timestamp</type> )
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Bin input into specified interval aligned with specified origin; see <xref linkend="functions-datetime-bin"/>
        </para>
        <para>
         <literal>date_bin('15 minutes', timestamp '2001-02-16 20:38:40', timestamp '2001-02-16 20:05:00')</literal>
         <returnvalue>2001-02-16 20:35:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>date_part</primary>
         </indexterm>
         <function>date_part</function> ( <type>text</type>, <type>timestamp</type> )
         <returnvalue>double precision</returnvalue>
        </para>
        <para>
         Get timestamp subfield (equivalent to <function>extract</function>);
         see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>date_part('hour', timestamp '2001-02-16 20:38:40')</literal>
         <returnvalue>20</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>date_part</function> ( <type>text</type>, <type>interval</type> )
         <returnvalue>double precision</returnvalue>
        </para>
        <para>
         Get interval subfield (equivalent to <function>extract</function>);
         see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>date_part('month', interval '2 years 3 months')</literal>
         <returnvalue>3</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>date_subtract</primary>
         </indexterm>
         <function>date_subtract</function> ( <type>timestamp with time zone</type>, <type>interval</type> <optional>, <type>text</type> </optional> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Subtract an <type>interval</type> from a <type>timestamp with time
         zone</type>, computing times of day and daylight-savings adjustments
         according to the time zone named by the third argument, or the
         current <xref linkend="guc-timezone"/> setting if that is omitted.
         The form with two arguments is equivalent to the <type>timestamp with
         time zone</type> <literal>-</literal> <type>interval</type> operator.
        </para>
        <para>
         <literal>date_subtract('2021-11-01 00:00:00+01'::timestamptz, '1 day'::interval, 'Europe/Warsaw')</literal>
         <returnvalue>2021-10-30 22:00:00+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>date_trunc</primary>
         </indexterm>
         <function>date_trunc</function> ( <type>text</type>, <type>timestamp</type> )
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Truncate to specified precision; see <xref linkend="functions-datetime-trunc"/>
        </para>
        <para>
         <literal>date_trunc('hour', timestamp '2001-02-16 20:38:40')</literal>
         <returnvalue>2001-02-16 20:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>date_trunc</function> ( <type>text</type>, <type>timestamp with time zone</type>, <type>text</type> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Truncate to specified precision in the specified time zone; see
         <xref linkend="functions-datetime-trunc"/>
        </para>
        <para>
         <literal>date_trunc('day', timestamptz '2001-02-16 20:38:40+00', 'Australia/Sydney')</literal>
         <returnvalue>2001-02-16 13:00:00+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>date_trunc</function> ( <type>text</type>, <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Truncate to specified precision; see
         <xref linkend="functions-datetime-trunc"/>
        </para>
        <para>
         <literal>date_trunc('hour', interval '2 days 3 hours 40 minutes')</literal>
         <returnvalue>2 days 03:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>extract</primary>
         </indexterm>
         <function>extract</function> ( <parameter>field</parameter> <literal>from</literal> <type>timestamp</type> )
         <returnvalue>numeric</returnvalue>
        </para>
        <para>
         Get timestamp subfield; see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>extract(hour from timestamp '2001-02-16 20:38:40')</literal>
         <returnvalue>20</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>extract</function> ( <parameter>field</parameter> <literal>from</literal> <type>interval</type> )
         <returnvalue>numeric</returnvalue>
        </para>
        <para>
         Get interval subfield; see <xref linkend="functions-datetime-extract"/>
        </para>
        <para>
         <literal>extract(month from interval '2 years 3 months')</literal>
         <returnvalue>3</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>isfinite</primary>
         </indexterm>
         <function>isfinite</function> ( <type>date</type> )
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Test for finite date (not +/-infinity)
        </para>
        <para>
         <literal>isfinite(date '2001-02-16')</literal>
         <returnvalue>true</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>isfinite</function> ( <type>timestamp</type> )
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Test for finite timestamp (not +/-infinity)
        </para>
        <para>
         <literal>isfinite(timestamp 'infinity')</literal>
         <returnvalue>false</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>isfinite</function> ( <type>interval</type> )
         <returnvalue>boolean</returnvalue>
        </para>
        <para>
         Test for finite interval (not +/-infinity)
        </para>
        <para>
         <literal>isfinite(interval '4 hours')</literal>
         <returnvalue>true</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm id="function-justify-days">
          <primary>justify_days</primary>
         </indexterm>
         <function>justify_days</function> ( <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Adjust interval, converting 30-day time periods to months
        </para>
        <para>
         <literal>justify_days(interval '1 year 65 days')</literal>
         <returnvalue>1 year 2 mons 5 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm id="function-justify-hours">
          <primary>justify_hours</primary>
         </indexterm>
         <function>justify_hours</function> ( <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Adjust interval, converting 24-hour time periods to days
        </para>
        <para>
         <literal>justify_hours(interval '50 hours 10 minutes')</literal>
         <returnvalue>2 days 02:10:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>justify_interval</primary>
         </indexterm>
         <function>justify_interval</function> ( <type>interval</type> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Adjust interval using <function>justify_days</function>
         and <function>justify_hours</function>, with additional sign
         adjustments
        </para>
        <para>
         <literal>justify_interval(interval '1 mon -1 hour')</literal>
         <returnvalue>29 days 23:00:00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>localtime</primary>
         </indexterm>
         <function>localtime</function>
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Current time of day;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtime</literal>
         <returnvalue>14:39:53.662522</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>localtime</function> ( <type>integer</type> )
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Current time of day, with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtime(0)</literal>
         <returnvalue>14:39:53</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>localtimestamp</primary>
         </indexterm>
         <function>localtimestamp</function>
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtimestamp</literal>
         <returnvalue>2019-12-23 14:39:53.662522</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <function>localtimestamp</function> ( <type>integer</type> )
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Current date and time (start of current
         transaction), with limited precision;
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>localtimestamp(2)</literal>
         <returnvalue>2019-12-23 14:39:53.66</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_date</primary>
         </indexterm>
         <function>make_date</function> ( <parameter>year</parameter> <type>int</type>,
         <parameter>month</parameter> <type>int</type>,
         <parameter>day</parameter> <type>int</type> )
         <returnvalue>date</returnvalue>
        </para>
        <para>
         Create date from year, month and day fields
         (negative years signify BC)
        </para>
        <para>
         <literal>make_date(2013, 7, 15)</literal>
         <returnvalue>2013-07-15</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature"><indexterm>
          <primary>make_interval</primary>
         </indexterm>
         <function>make_interval</function> ( <optional> <parameter>years</parameter> <type>int</type>
         <optional>, <parameter>months</parameter> <type>int</type>
         <optional>, <parameter>weeks</parameter> <type>int</type>
         <optional>, <parameter>days</parameter> <type>int</type>
         <optional>, <parameter>hours</parameter> <type>int</type>
         <optional>, <parameter>mins</parameter> <type>int</type>
         <optional>, <parameter>secs</parameter> <type>double precision</type>
         </optional></optional></optional></optional></optional></optional></optional> )
         <returnvalue>interval</returnvalue>
        </para>
        <para>
         Create interval from years, months, weeks, days, hours, minutes and
         seconds fields, each of which can default to zero
        </para>
        <para>
         <literal>make_interval(days =&gt; 10)</literal>
         <returnvalue>10 days</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_time</primary>
         </indexterm>
         <function>make_time</function> ( <parameter>hour</parameter> <type>int</type>,
         <parameter>min</parameter> <type>int</type>,
         <parameter>sec</parameter> <type>double precision</type> )
         <returnvalue>time</returnvalue>
        </para>
        <para>
         Create time from hour, minute and seconds fields
        </para>
        <para>
         <literal>make_time(8, 15, 23.5)</literal>
         <returnvalue>08:15:23.5</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_timestamp</primary>
         </indexterm>
         <function>make_timestamp</function> ( <parameter>year</parameter> <type>int</type>,
         <parameter>month</parameter> <type>int</type>,
         <parameter>day</parameter> <type>int</type>,
         <parameter>hour</parameter> <type>int</type>,
         <parameter>min</parameter> <type>int</type>,
         <parameter>sec</parameter> <type>double precision</type> )
         <returnvalue>timestamp</returnvalue>
        </para>
        <para>
         Create timestamp from year, month, day, hour, minute and seconds fields
         (negative years signify BC)
        </para>
        <para>
         <literal>make_timestamp(2013, 7, 15, 8, 15, 23.5)</literal>
         <returnvalue>2013-07-15 08:15:23.5</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>make_timestamptz</primary>
         </indexterm>
         <function>make_timestamptz</function> ( <parameter>year</parameter> <type>int</type>,
         <parameter>month</parameter> <type>int</type>,
         <parameter>day</parameter> <type>int</type>,
         <parameter>hour</parameter> <type>int</type>,
         <parameter>min</parameter> <type>int</type>,
         <parameter>sec</parameter> <type>double precision</type>
         <optional>, <parameter>timezone</parameter> <type>text</type> </optional> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Create timestamp with time zone from year, month, day, hour, minute
         and seconds fields (negative years signify BC).
         If <parameter>timezone</parameter> is not
         specified, the current time zone is used; the examples assume the
         session time zone is <literal>Europe/London</literal>
        </para>
        <para>
         <literal>make_timestamptz(2013, 7, 15, 8, 15, 23.5)</literal>
         <returnvalue>2013-07-15 08:15:23.5+01</returnvalue>
        </para>
        <para>
         <literal>make_timestamptz(2013, 7, 15, 8, 15, 23.5, 'America/New_York')</literal>
         <returnvalue>2013-07-15 13:15:23.5+01</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>now</primary>
         </indexterm>
         <function>now</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>now()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>statement_timestamp</primary>
         </indexterm>
         <function>statement_timestamp</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current statement);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>statement_timestamp()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>timeofday</primary>
         </indexterm>
         <function>timeofday</function> ( )
         <returnvalue>text</returnvalue>
        </para>
        <para>
         Current date and time
         (like <function>clock_timestamp</function>, but as a <type>text</type> string);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>timeofday()</literal>
         <returnvalue>Mon Dec 23 14:39:53.662522 2019 EST</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>transaction_timestamp</primary>
         </indexterm>
         <function>transaction_timestamp</function> ( )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Current date and time (start of current transaction);
         see <xref linkend="functions-datetime-current"/>
        </para>
        <para>
         <literal>transaction_timestamp()</literal>
         <returnvalue>2019-12-23 14:39:53.662522-05</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
          <primary>to_timestamp</primary>
         </indexterm>
         <function>to_timestamp</function> ( <type>double precision</type> )
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to
         timestamp with time zone
        </para>
        <para>
         <literal>to_timestamp(1284352323)</literal>
         <returnvalue>2010-09-13 04:32:03+00</returnvalue>
        </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    <indexterm>
      <primary>OVERLAPS</primary>
    </indexterm>
    In addition to these functions, the SQL <literal>OVERLAPS</literal> operator is
    supported:
<synopsis>
(<replaceable>start1</replaceable>, <replaceable>end1</replaceable>) OVERLAPS (<replaceable>start2</replaceable>, <replaceable>end2</replaceable>)
(<replaceable>start1</replaceable>, <replaceable>length1</replaceable>) OVERLAPS (<replaceable>start2</replaceable>, <replaceable>length2</replaceable>)
</synopsis>
    This expression yields true when two time periods (defined by their
    endpoints) overlap, false when they do not overlap.  The endpoints
    can be specified as pairs of dates, times, or time stamps; or as
    a date, time, or time stamp followed by an interval.  When a pair
    of values is provided, either the start or the end can be written
    first; <literal>OVERLAPS</literal> automatically takes the earlier value
    of the pair as the start.  Each time period is considered to
    represent the half-open interval <replaceable>start</replaceable> <literal>&lt;=</literal>
    <replaceable>time</replaceable> <literal>&lt;</literal> <replaceable>end</replaceable>, unless
    <replaceable>start</replaceable> and <replaceable>end</replaceable> are equal in which case it
    represents that single time instant.  This means for instance that two
    time periods with only an endpoint in common do not overlap.
   </para>

<screen>
SELECT (DATE '2001-02-16', DATE '2001-12-21') OVERLAPS
       (DATE '2001-10-30', DATE '2002-10-30');
<lineannotation>Result: </lineannotation><computeroutput>true</computeroutput>
SELECT (DATE '2001-02-16', INTERVAL '100 days') OVERLAPS
       (DATE '2001-10-30', DATE '2002-10-30');
<lineannotation>Result: </lineannotation><computeroutput>false</computeroutput>
SELECT (DATE '2001-10-29', DATE '2001-10-30') OVERLAPS
       (DATE '2001-10-30', DATE '2001-10-31');
<lineannotation>Result: </lineannotation><computeroutput>false</computeroutput>
SELECT (DATE '2001-10-30', DATE '2001-10-30') OVERLAPS
       (DATE '2001-10-30', DATE '2001-10-31');
<lineannotation>Result: </lineannotation><computeroutput>true</computeroutput>
</screen>

  <para>
   When adding an <type>interval</type> value to (or subtracting an
   <type>interval</type> value from) a <type>timestamp</type>
   or <type>timestamp with time zone</type> value, the months, days, and
   microseconds fields of the <type>interval</type> value are handled in turn.
   First, a nonzero months field advances or decrements the date of the
   timestamp by the indicated number of months, keeping the day of month the
   same unless it would be past the end of the new month, in which case the
   last day of that month is used.  (For example, March 31 plus 1 month
   becomes April 30, but March 31 plus 2 months becomes May 31.)
   Then the days field advances or decrements the date of the timestamp by
   the indicated number of days.  In both these steps the local time of day
   is kept the same.  Finally, if there is a nonzero microseconds field, it
   is added or subtracted literally.
   When doing arithmetic on a <type>timestamp with time zone</type> value in
   a time zone that recognizes DST, this means that adding or subtracting
   (say) <literal>interval '1 day'</literal> does not necessarily have the
   same result as adding or subtracting <literal>interval '24
   hours'</literal>.
   For example, with the session time zone set
   to <literal>America/Denver</literal>:
<screen>
SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '1 day';
<lineannotation>Result: </lineannotation><computeroutput>2005-04-03 12:00:00-06</computeroutput>
SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '24 hours';
<lineannotation>Result: </lineannotation><computeroutput>2005-04-03 13:00:00-06</computeroutput>
</screen>
   This happens because an hour was skipped due to a change in daylight saving
   time at <literal>2005-04-03 02:00:00</literal> in time zone
   <literal>America/Denver</literal>.
  </para>

  <para>
   Note there can be ambiguity in the <literal>months</literal> field returned by
   <function>age</function> because different months have different numbers of
   days.  <productname>PostgreSQL</productname>'s approach uses the month from the
   earlier of the two dates when calculating partial months.  For example,
   <literal>age('2004-06-01', '2004-04-30')</literal> uses April to yield
   <literal>1 mon 1 day</literal>, while using May would yield <literal>1 mon 2
   days</literal> because May has 31 days, while April has only 30.
  </para>

  <para>
   Subtraction of dates and timestamps can also be complex.  One conceptually
   simple way to perform subtraction is to convert each value to a number
   of seconds using <literal>EXTRACT(EPOCH FROM ...)</literal>, then subtract the
   results; this produces the
   number of <emphasis>seconds</emphasis> between the two values.  This will adjust
   for the number of days in each month, timezone changes, and daylight
   saving time adjustments.  Subtraction of date or timestamp
   values with the <quote><literal>-</literal></quote> operator
   returns the number of days (24-hours) and hours/minutes/seconds
   between the values, making the same adjustments.  The <function>age</function>
   function returns years, months, days, and hours/minutes/seconds,
   performing field-by-field subtraction and then adjusting for negative
   field values.  The following queries illustrate the differences in these
   approaches.  The sample results were produced with <literal>timezone
   = 'US/Eastern'</literal>; there is a daylight saving time change between the
   two dates used:
  </para>

<screen>
SELECT EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
       EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00');
<lineannotation>Result: </lineannotation><computeroutput>10537200.000000</computeroutput>
SELECT (EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
        EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00'))
        / 60 / 60 / 24;
<lineannotation>Result: </lineannotation><computeroutput>121.9583333333333333</computeroutput>
SELECT timestamptz '2013-07-01 12:00:00' - timestamptz '2013-03-01 12:00:00';
<lineannotation>Result: </lineannotation><computeroutput>121 days 23:00:00</computeroutput>
SELECT age(timestamptz '2013-07-01 12:00:00', timestamptz '2013-03-01 12:00:00');
<lineannotation>Result: </lineannotation><computeroutput>4 mons</computeroutput>
</screen>

  <sect2 id="functions-datetime-extract">
   <title><function>EXTRACT</function>, <function>date_part</function></title>

   <indexterm>
    <primary>date_part</primary>
   </indexterm>
   <indexterm>
    <primary>extract</primary>
   </indexterm>

<synopsis>
EXTRACT(<replaceable>field</replaceable> FROM <replaceable>source</replaceable>)
</synopsis>

   <para>
    The <function>extract</function> function retrieves subfields
    such as year or hour from date/time values.
    <replaceable>source</replaceable> must be a value expression of
    type <type>timestamp</type>, <type>date</type>, <type>time</type>,
    or <type>interval</type>.  (Timestamps and times can be with or
    without time zone.)
    <replaceable>field</replaceable> is an identifier or
    string that selects what field to extract from the source value.
    Not all fields are valid for every input data type; for example, fields
    smaller than a day cannot be extracted from a <type>date</type>, while
    fields of a day or more cannot be extracted from a <type>time</type>.
    The <function>extract</function> function returns values of type
    <type>numeric</type>.
   </para>

   <para>
    The following are valid field names:

    <!-- alphabetical -->
    <variablelist>
     <varlistentry>
      <term><literal>century</literal></term>
      <listitem>
       <para>
        The century; for <type>interval</type> values, the year field
        divided by 100
       </para>

<screen>
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>21</computeroutput>
SELECT EXTRACT(CENTURY FROM DATE '0001-01-01 AD');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
SELECT EXTRACT(CENTURY FROM DATE '0001-12-31 BC');
<lineannotation>Result: </lineannotation><computeroutput>-1</computeroutput>
SELECT EXTRACT(CENTURY FROM INTERVAL '2001 years');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>day</literal></term>
      <listitem>
       <para>
        The day of the month (1&ndash;31); for <type>interval</type>
        values, the number of days
       </para>

<screen>
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute');
<lineannotation>Result: </lineannotation><computeroutput>40</computeroutput>
</screen>

      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>decade</literal></term>
      <listitem>
       <para>
        The year field divided by 10
       </para>

<screen>
SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>200</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>dow</literal></term>
      <listitem>
       <para>
        The day of the week as Sunday (<literal>0</literal>) to
        Saturday (<literal>6</literal>)
       </para>

<screen>
SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>5</computeroutput>
</screen>
       <para>
        Note that <function>extract</function>'s day of the week numbering
        differs from that of the <function>to_char(...,
        'D')</function> function.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>doy</literal></term>
      <listitem>
       <para>
        The day of the year (1&ndash;365/366)
       </para>

<screen>
SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>47</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>epoch</literal></term>
      <listitem>
       <para>
        For <type>timestamp with time zone</type> values, the
        number of seconds since 1970-01-01 00:00:00 UTC (negative for
        timestamps before that);
        for <type>date</type> and <type>timestamp</type> values, the
        nominal number of seconds since 1970-01-01 00:00:00,
        without regard to timezone or daylight-savings rules;
        for <type>interval</type> values, the total number
        of seconds in the interval
       </para>

<screen>
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08');
<lineannotation>Result: </lineannotation><computeroutput>982384720.120000</computeroutput>
SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40.12');
<lineannotation>Result: </lineannotation><computeroutput>982355920.120000</computeroutput>
SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
<lineannotation>Result: </lineannotation><computeroutput>442800.000000</computeroutput>
</screen>

       <para>
        You can convert an epoch value back to a <type>timestamp with time zone</type>
        with <function>to_timestamp</function>:
       </para>
<screen>
SELECT to_timestamp(982384720.12);
<lineannotation>Result: </lineannotation><computeroutput>2001-02-17 04:38:40.12+00</computeroutput>
</screen>

       <para>
        Beware that applying <function>to_timestamp</function> to an epoch
        extracted from a <type>date</type> or <type>timestamp</type> value
        could produce a misleading result: the result will effectively
        assume that the original value had been given in UTC, which might
        not be the case.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>hour</literal></term>
      <listitem>
       <para>
        The hour field (0&ndash;23 in timestamps, unrestricted in
        intervals)
       </para>

<screen>
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>isodow</literal></term>
      <listitem>
       <para>
        The day of the week as Monday (<literal>1</literal>) to
        Sunday (<literal>7</literal>)
       </para>

<screen>
SELECT EXTRACT(ISODOW FROM TIMESTAMP '2001-02-18 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>7</computeroutput>
</screen>
       <para>
        This is identical to <literal>dow</literal> except for Sunday.  This
        matches the <acronym>ISO</acronym> 8601 day of the week numbering.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>isoyear</literal></term>
      <listitem>
       <para>
        The <acronym>ISO</acronym> 8601 week-numbering year that the date
        falls in
       </para>

<screen>
SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-01');
<lineannotation>Result: </lineannotation><computeroutput>2005</computeroutput>
SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-02');
<lineannotation>Result: </lineannotation><computeroutput>2006</computeroutput>
</screen>

       <para>
        Each <acronym>ISO</acronym> 8601 week-numbering year begins with the
        Monday of the week containing the 4th of January, so in early
        January or late December the <acronym>ISO</acronym> year may be
        different from the Gregorian year.  See the <literal>week</literal>
        field for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>julian</literal></term>
      <listitem>
       <para>
        The <firstterm>Julian Date</firstterm> corresponding to the
        date or timestamp.  Timestamps
        that are not local midnight result in a fractional value.  See
        <xref linkend="datetime-julian-dates"/> for more information.
       </para>

<screen>
SELECT EXTRACT(JULIAN FROM DATE '2006-01-01');
<lineannotation>Result: </lineannotation><computeroutput>2453737</computeroutput>
SELECT EXTRACT(JULIAN FROM TIMESTAMP '2006-01-01 12:00');
<lineannotation>Result: </lineannotation><computeroutput>2453737.50000000000000000000</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>microseconds</literal></term>
      <listitem>
       <para>
        The seconds field, including fractional parts, multiplied by 1
        000 000;  note that this includes full seconds
       </para>

<screen>
SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28500000</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>millennium</literal></term>
      <listitem>
       <para>
        The millennium; for <type>interval</type> values, the year field
        divided by 1000
       </para>

<screen>
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>
SELECT EXTRACT(MILLENNIUM FROM INTERVAL '2001 years');
<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
</screen>

       <para>
        Years in the 1900s are in the second millennium.
        The third millennium started January 1, 2001.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>milliseconds</literal></term>
      <listitem>
       <para>
        The seconds field, including fractional parts, multiplied by
        1000.  Note that this includes full seconds.
       </para>

<screen>
SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28500.000</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>minute</literal></term>
      <listitem>
       <para>
        The minutes field (0&ndash;59)
       </para>

<screen>
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>38</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>month</literal></term>
      <listitem>
       <para>
        The number of the month within the year (1&ndash;12);
        for <type>interval</type> values, the number of months modulo 12
        (0&ndash;11)
       </para>

<screen>
SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');
<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>
SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>quarter</literal></term>
      <listitem>
       <para>
        The quarter of the year (1&ndash;4) that the date is in;
        for <type>interval</type> values, the month field divided by 3
        plus 1
       </para>

<screen>
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
SELECT EXTRACT(QUARTER FROM INTERVAL '1 year 6 months');
<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>second</literal></term>
      <listitem>
       <para>
        The seconds field, including any fractional seconds
       </para>

<screen>
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>40.000000</computeroutput>
SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28.500000</computeroutput>
</screen>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>timezone</literal></term>
      <listitem>
       <para>
        The time zone offset from UTC, measured in seconds.  Positive values
        correspond to time zones east of UTC, negative values to
        zones west of UTC.  (Technically,
        <productname>PostgreSQL</productname> does not use UTC because
        leap seconds are not handled.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>timezone_hour</literal></term>
      <listitem>
       <para>
        The hour component of the time zone offset
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>timezone_minute</literal></term>
      <listitem>
       <para>
        The minute component of the time zone offset
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>week</literal></term>
      <listitem>
       <para>
        The number of the <acronym>ISO</acronym> 8601 week-numbering week of
        the year.  By definition, ISO weeks start on Mondays and the first
        week of a year contains January 4 of that year.  In other words, the
        first Thursday of a year is in week 1 of that year.
       </para>
       <para>
        In the ISO week-numbering system, it is possible for early-January
        dates to be part of the 52nd or 53rd week of the previous year, and for
        late-December dates to be part of the first week of the next year.
        For example, <literal>2005-01-01</literal> is part of the 53rd week of year
        2004, and <literal>2006-01-01</literal> is part of the 52nd week of year
        2005, while <literal>2012-12-31</literal> is part of the first week of 2013.
        It's recommended to use the <literal>isoyear</literal> field together with
        <literal>week</literal> to get consistent results.
       </para>

       <para>
        For <type>interval</type> values, the week field is simply the number
        of integral days divided by 7.
       </para>

<screen>
SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>7</computeroutput>
SELECT EXTRACT(WEEK FROM INTERVAL '13 days 24 hours');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>year</literal></term>
      <listitem>
       <para>
        The year field.  Keep in mind there is no <literal>0 AD</literal>, so subtracting
        <literal>BC</literal> years from <literal>AD</literal> years should be done with care.
       </para>

<screen>
SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001</computeroutput>
</screen>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>

   <para>
    When processing an <type>interval</type> value,
    the <function>extract</function> function produces field values that
    match the interpretation used by the interval output function.  This
    can produce surprising results if one starts with a non-normalized
    interval representation, for example:
<screen>
SELECT INTERVAL '80 minutes';
<lineannotation>Result: </lineannotation><computeroutput>01:20:00</computeroutput>
SELECT EXTRACT(MINUTES FROM INTERVAL '80 minutes');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
</screen>
   </para>

   <note>
    <para>
     When the input value is +/-Infinity, <function>extract</function> returns
     +/-Infinity for monotonically-increasing fields (<literal>epoch</literal>,
     <literal>julian</literal>, <literal>year</literal>, <literal>isoyear</literal>,
     <literal>decade</literal>, <literal>century</literal>, and <literal>millennium</literal>
     for <type>timestamp</type> inputs; <literal>epoch</literal>, <literal>hour</literal>,
     <literal>day</literal>, <literal>year</literal>, <literal>decade</literal>,
     <literal>century</literal>, and <literal>millennium</literal> for
     <type>interval</type> inputs).
     For other fields, NULL is returned.  <productname>PostgreSQL</productname>
     versions before 9.6 returned zero for all cases of infinite input.
    </para>
   </note>

   <para>
    The <function>extract</function> function is primarily intended
    for computational processing.  For formatting date/time values for
    display, see <xref linkend="functions-formatting"/>.
   </para>

   <para>
    The <function>date_part</function> function is modeled on the traditional
    <productname>Ingres</productname> equivalent to the
    <acronym>SQL</acronym>-standard function <function>extract</function>:
<synopsis>
date_part('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
</synopsis>
    Note that here the <replaceable>field</replaceable> parameter needs to
    be a string value, not a name.  The valid field names for
    <function>date_part</function> are the same as for
    <function>extract</function>.
    For historical reasons, the <function>date_part</function> function
    returns values of type <type>double precision</type>.  This can result in
    a loss of precision in certain uses.  Using <function>extract</function>
    is recommended instead.
   </para>

<screen>
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
SELECT date_part('hour', INTERVAL '4 hours 3 minutes');
<lineannotation>Result: </lineannotation><computeroutput>4</computeroutput>
</screen>

  </sect2>

  <sect2 id="functions-datetime-trunc">
   <title><function>date_trunc</function></title>

   <indexterm>
    <primary>date_trunc</primary>
   </indexterm>

   <para>
    The function <function>date_trunc</function> is conceptually
    similar to the <function>trunc</function> function for numbers.
   </para>

   <para>
<synopsis>
date_trunc(<replaceable>field</replaceable>, <replaceable>source</replaceable> <optional>, <replaceable>time_zone</replaceable> </optional>)
</synopsis>
    <replaceable>source</replaceable> is a value expression of type
    <type>timestamp</type>, <type>timestamp with time zone</type>,
    or <type>interval</type>.
    (Values of type <type>date</type> and
    <type>time</type> are cast automatically to <type>timestamp</type> or
    <type>interval</type>, respectively.)
    <replaceable>field</replaceable> selects to which precision to
    truncate the input value.  The return value is likewise of type
    <type>timestamp</type>, <type>timestamp with time zone</type>,
    or <type>interval</type>,
    and it has all fields that are less significant than the
    selected one set to zero (or one, for day and month).
   </para>

   <para>
    Valid values for <replaceable>field</replaceable> are:
    <simplelist>
     <member><literal>microseconds</literal></member>
     <member><literal>milliseconds</literal></member>
     <member><literal>second</literal></member>
     <member><literal>minute</literal></member>
     <member><literal>hour</literal></member>
     <member><literal>day</literal></member>
     <member><literal>week</literal></member>
     <member><literal>month</literal></member>
     <member><literal>quarter</literal></member>
     <member><literal>year</literal></member>
     <member><literal>decade</literal></member>
     <member><literal>century</literal></member>
     <member><literal>millennium</literal></member>
    </simplelist>
   </para>

   <para>
    When the input value is of type <type>timestamp with time zone</type>,
    the truncation is performed with respect to a particular time zone;
    for example, truncation to <literal>day</literal> produces a value that
    is midnight in that zone.  By default, truncation is done with respect
    to the current <xref linkend="guc-timezone"/> setting, but the
    optional <replaceable>time_zone</replaceable> argument can be provided
    to specify a different time zone.  The time zone name can be specified
    in any of the ways described in <xref linkend="datatype-timezones"/>.
   </para>

   <para>
    A time zone cannot be specified when processing <type>timestamp without
    time zone</type> or <type>interval</type> inputs.  These are always
    taken at face value.
   </para>

   <para>
    Examples (assuming the local time zone is <literal>America/New_York</literal>):
<screen>
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 20:00:00</computeroutput>
SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-01-01 00:00:00</computeroutput>
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 00:00:00-05</computeroutput>
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00', 'Australia/Sydney');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 08:00:00-05</computeroutput>
SELECT date_trunc('hour', INTERVAL '3 days 02:47:33');
<lineannotation>Result: </lineannotation><computeroutput>3 days 02:00:00</computeroutput>
</screen>
   </para>
  </sect2>

  <sect2 id="functions-datetime-bin">
   <title><function>date_bin</function></title>

   <indexterm>
    <primary>date_bin</primary>
   </indexterm>

   <para>
    The function <function>date_bin</function> <quote>bins</quote> the input
    timestamp into the specified interval (the <firstterm>stride</firstterm>)
    aligned with a specified origin.
   </para>

   <para>
<synopsis>
date_bin(<replaceable>stride</replaceable>, <replaceable>source</replaceable>, <replaceable>origin</replaceable>)
</synopsis>
    <replaceable>source</replaceable> is a value expression of type
    <type>timestamp</type> or <type>timestamp with time zone</type>.  (Values
    of type <type>date</type> are cast automatically to
    <type>timestamp</type>.)  <replaceable>stride</replaceable> is a value
    expression of type <type>interval</type>.  The return value is likewise
    of type <type>timestamp</type> or <type>timestamp with time zone</type>,
    and it marks the beginning of the bin into which the
    <replaceable>source</replaceable> is placed.
   </para>

   <para>
    Examples:
<screen>
SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-01-01');
<lineannotation>Result: </lineannotation><computeroutput>2020-02-11 15:30:00</computeroutput>
SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-01-01 00:02:30');
<lineannotation>Result: </lineannotation><computeroutput>2020-02-11 15:32:30</computeroutput>
</screen>
   </para>

   <para>
    In the case of full units (1 minute, 1 hour, etc.), it gives the same result as
    the analogous <function>date_trunc</function> call, but the difference is
    that <function>date_bin</function> can truncate to an arbitrary interval.
   </para>

   <para>
    The <parameter>stride</parameter> interval must be greater than zero and
    cannot contain units of month or larger.
   </para>
  </sect2>

  <sect2 id="functions-datetime-zoneconvert">
   <title><literal>AT TIME ZONE</literal> and <literal>AT LOCAL</literal></title>

   <indexterm>
    <primary>time zone</primary>
    <secondary>conversion</secondary>
   </indexterm>

   <indexterm>
    <primary>AT TIME ZONE</primary>
   </indexterm>

   <indexterm>
    <primary>AT LOCAL</primary>
   </indexterm>

   <para>
    The <literal>AT TIME ZONE</literal> operator converts time
    stamp <emphasis>without</emphasis> time zone to/from
    time stamp <emphasis>with</emphasis> time zone, and
    <type>time with time zone</type> values to different time
    zones. <xref linkend="functions-datetime-zoneconvert-table"/> shows its
    variants.
   </para>

    <table id="functions-datetime-zoneconvert-table">
     <title><literal>AT TIME ZONE</literal> and <literal>AT LOCAL</literal> Variants</title>
     <tgroup cols="1">
      <thead>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         Operator
        </para>
        <para>
         Description
        </para>
        <para>
         Example(s)
        </para></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp without time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Converts given time stamp <emphasis>without</emphasis> time zone to
         time stamp <emphasis>with</emphasis> time zone, assuming the given
         value is in the named time zone.
        </para>
        <para>
         <literal>timestamp '2001-02-16 20:38:40' at time zone 'America/Denver'</literal>
         <returnvalue>2001-02-17 03:38:40+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp without time zone</type> <literal>AT LOCAL</literal>
         <returnvalue>timestamp with time zone</returnvalue>
        </para>
        <para>
         Converts given time stamp <emphasis>without</emphasis> time zone to
         time stamp <emphasis>with</emphasis> the session's
         <varname>TimeZone</varname> value as time zone.
        </para>
        <para>
         <literal>timestamp '2001-02-16 20:38:40' at local</literal>
         <returnvalue>2001-02-17 03:38:40+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp with time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
         <returnvalue>timestamp without time zone</returnvalue>
        </para>
        <para>
         Converts given time stamp <emphasis>with</emphasis> time zone to
         time stamp <emphasis>without</emphasis> time zone, as the time would
         appear in that zone.
        </para>
        <para>
         <literal>timestamp with time zone '2001-02-16 20:38:40-05' at time zone 'America/Denver'</literal>
         <returnvalue>2001-02-16 18:38:40</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>timestamp with time zone</type> <literal>AT LOCAL</literal>
         <returnvalue>timestamp without time zone</returnvalue>
        </para>
        <para>
         Converts given time stamp <emphasis>with</emphasis> time zone to
         time stamp <emphasis>without</emphasis> time zone, as the time would
         appear with the session's <varname>TimeZone</varname> value as time zone.
        </para>
        <para>
         <literal>timestamp with time zone '2001-02-16 20:38:40-05' at local</literal>
         <returnvalue>2001-02-16 18:38:40</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time with time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
         <returnvalue>time with time zone</returnvalue>
        </para>
        <para>
         Converts given time <emphasis>with</emphasis> time zone to a new time
         zone.  Since no date is supplied, this uses the currently active UTC
         offset for the named destination zone.
        </para>
        <para>
         <literal>time with time zone '05:34:17-05' at time zone 'UTC'</literal>
         <returnvalue>10:34:17+00</returnvalue>
        </para></entry>
       </row>

       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <type>time with time zone</type> <literal>AT LOCAL</literal>
         <returnvalue>time with time zone</returnvalue>
        </para>
        <para>
         Converts given time <emphasis>with</emphasis> time zone to a new time
         zone.  Since no date is supplied, this uses the currently active UTC
         offset for the session's <varname>TimeZone</varname> value.
        </para>
        <para>
         Assuming the session's <varname>TimeZone</varname> is set to <literal>UTC</literal>:
        </para>
        <para>
         <literal>time with time zone '05:34:17-05' at local</literal>
         <returnvalue>10:34:17+00</returnvalue>
        </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    In these expressions, the desired time zone <replaceable>zone</replaceable> can be
    specified either as a text value (e.g., <literal>'America/Los_Angeles'</literal>)
    or as an interval (e.g., <literal>INTERVAL '-08:00'</literal>).
    In the text case, a time zone name can be specified in any of the ways
    described in <xref linkend="datatype-timezones"/>.
    The interval case is only useful for zones that have fixed offsets from
    UTC, so it is not very common in practice.
   </para>

   <para>
    The syntax <literal>AT LOCAL</literal> may be used as shorthand for
    <literal>AT TIME ZONE <replaceable>local</replaceable></literal>, where
    <replaceable>local</replaceable> is the session's
    <varname>TimeZone</varname> value.
   </para>

   <para>
    Examples (assuming the current <xref linkend="guc-timezone"/> setting
    is <literal>America/Los_Angeles</literal>):
<screen>
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 19:38:40-08</computeroutput>
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 05:38:40</computeroutput>
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT LOCAL;
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 17:38:40</computeroutput>
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE '+05';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 20:38:40</computeroutput>
SELECT TIME WITH TIME ZONE '20:38:40-05' AT LOCAL;
<lineannotation>Result: </lineannotation><computeroutput>17:38:40</computeroutput>
</screen>
    The first example adds a time zone to a value that lacks it, and
    displays the value using the current <varname>TimeZone</varname>
    setting.  The second example shifts the time stamp with time zone value
    to the specified time zone, and returns the value without a time zone.
    This allows storage and display of values different from the current
    <varname>TimeZone</varname> setting.  The third example converts
    Tokyo time to Chicago time.  The fourth example shifts the time stamp
    with time zone value to the time zone currently specified by the
    <varname>TimeZone</varname> setting and returns the value without a
    time zone.  The fifth example demonstrates that the sign in a POSIX-style
    time zone specification has the opposite meaning of the sign in an
    ISO-8601 datetime literal, as described in <xref linkend="datatype-timezones"/>
    and <xref linkend="datetime-appendix"/>.
   </para>

   <para>
    The sixth example is a cautionary tale. Due to the fact that there is no
    date associated with the input value, the conversion is made using the
    current date of the session. Therefore, this static example may show a wrong
    result depending on the time of the year it is viewed because
    <literal>'America/Los_Angeles'</literal> observes Daylight Savings Time.
   </para>

   <para>
    The function <literal><function>timezone</function>(<replaceable>zone</replaceable>,
    <replaceable>timestamp</replaceable>)</literal> is equivalent to the SQL-conforming construct
    <literal><replaceable>timestamp</replaceable> AT TIME ZONE
    <replaceable>zone</replaceable></literal>.
   </para>

   <para>
    The function <literal><function>timezone</function>(<replaceable>zone</replaceable>,
    <replaceable>time</replaceable>)</literal> is equivalent to the SQL-conforming construct
    <literal><replaceable>time</replaceable> AT TIME ZONE
    <replaceable>zone</replaceable></literal>.
   </para>

   <para>
    The function <literal><function>timezone</function>(<replaceable>timestamp</replaceable>)</literal>
    is equivalent to the SQL-conforming construct <literal><replaceable>timestamp</replaceable>
    AT LOCAL</literal>.
   </para>

   <para>
    The function <literal><function>timezone</function>(<replaceable>time</replaceable>)</literal>
    is equivalent to the SQL-conforming construct <literal><replaceable>time</replaceable>
    AT LOCAL</literal>.
   </para>
  </sect2>

  <sect2 id="functions-datetime-current">
   <title>Current Date/Time</title>

   <indexterm>
    <primary>date</primary>
    <secondary>current</secondary>
   </indexterm>

   <indexterm>
    <primary>time</primary>
    <secondary>current</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> provides a number of functions
    that return values related to the current date and time.  These
    SQL-standard functions all return values based on the start time of
    the current transaction:
<synopsis>
CURRENT_DATE
CURRENT_TIME
CURRENT_TIMESTAMP
CURRENT_TIME(<replaceable>precision</replaceable>)
CURRENT_TIMESTAMP(<replaceable>precision</replaceable>)
LOCALTIME
LOCALTIMESTAMP
LOCALTIME(<replaceable>precision</replaceable>)
LOCALTIMESTAMP(<replaceable>precision</replaceable>)
</synopsis>
    </para>

    <para>
     <function>CURRENT_TIME</function> and
     <function>CURRENT_TIMESTAMP</function> deliver values with time zone;
     <function>LOCALTIME</function> and
     <function>LOCALTIMESTAMP</function> deliver values without time zone.
    </para>

    <para>
     <function>CURRENT_TIME</function>,
     <function>CURRENT_TIMESTAMP</function>,
     <function>LOCALTIME</function>, and
     <function>LOCALTIMESTAMP</function>
     can optionally take
     a precision parameter, which causes the result to be rounded
     to that many fractional digits in the seconds field.  Without a precision parameter,
     the result is given to the full available precision.
    </para>

   <para>
    Some examples:
<screen>
SELECT CURRENT_TIME;
<lineannotation>Result: </lineannotation><computeroutput>14:39:53.662522-05</computeroutput>
SELECT CURRENT_DATE;
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23</computeroutput>
SELECT CURRENT_TIMESTAMP;
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23 14:39:53.662522-05</computeroutput>
SELECT CURRENT_TIMESTAMP(2);
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23 14:39:53.66-05</computeroutput>
SELECT LOCALTIMESTAMP;
<lineannotation>Result: </lineannotation><computeroutput>2019-12-23 14:39:53.662522</computeroutput>
</screen>
   </para>

   <para>
    Since these functions return
    the start time of the current transaction, their values do not
    change during the transaction. This is considered a feature:
    the intent is to allow a single transaction to have a consistent
    notion of the <quote>current</quote> time, so that multiple
    modifications within the same transaction bear the same
    time stamp.
   </para>

   <note>
    <para>
     Other database systems might advance these values more
     frequently.
    </para>
   </note>

   <para>
    <productname>PostgreSQL</productname> also provides functions that
    return the start time of the current statement, as well as the actual
    current time at the instant the function is called.  The complete list
    of non-SQL-standard time functions is:
<synopsis>
transaction_timestamp()
statement_timestamp()
clock_timestamp()
timeofday()
now()
</synopsis>
   </para>

   <para>
    <function>transaction_timestamp()</function> is equivalent to
    <function>CURRENT_TIMESTAMP</function>, but is named to clearly reflect
    what it returns.
    <function>statement_timestamp()</function> returns the start time of the current
    statement (more specifically, the time of receipt of the latest command
    message from the client).
    <function>statement_timestamp()</function> and <function>transaction_timestamp()</function>
    return the same value during the first statement of a transaction, but might
    differ during subsequent statements.
    <function>clock_timestamp()</function> returns the actual current time, and
    therefore its value changes even within a single SQL statement.
    <function>timeofday()</function> is a historical
    <productname>PostgreSQL</productname> function.  Like
    <function>clock_timestamp()</function>, it returns the actual current time,
    but as a formatted <type>text</type> string rather than a <type>timestamp
    with time zone</type> value.
    <function>now()</function> is a traditional <productname>PostgreSQL</productname>
    equivalent to <function>transaction_timestamp()</function>.
   </para>

   <para>
    All the date/time data types also accept the special literal value
    <literal>now</literal> to specify the current date and time (again,
    interpreted as the transaction start time).  Thus,
    the following three all return the same result:
<programlisting>
SELECT CURRENT_TIMESTAMP;
SELECT now();
SELECT TIMESTAMP 'now';  -- but see tip below
</programlisting>
   </para>

    <tip>
     <para>
      Do not use the third form when specifying a value to be evaluated later,
      for example in a <literal>DEFAULT</literal> clause for a table column.
      The system will convert <literal>now</literal>
      to a <type>timestamp</type> as soon as the constant is parsed, so that when
      the default value is needed,
      the time of the table creation would be used!  The first two
      forms will not be evaluated until the default value is used,
      because they are function calls.  Thus they will give the desired
      behavior of defaulting to the time of row insertion.
      (See also <xref linkend="datatype-datetime-special-values"/>.)
     </para>
    </tip>
  </sect2>

  <sect2 id="functions-datetime-delay">
   <title>Delaying Execution</title>

   <indexterm>
    <primary>pg_sleep</primary>
   </indexterm>
   <indexterm>
    <primary>pg_sleep_for</primary>
   </indexterm>
   <indexterm>
    <primary>pg_sleep_until</primary>
   </indexterm>
   <indexterm>
    <primary>sleep</primary>
   </indexterm>
   <indexterm>
    <primary>delay</primary>
   </indexterm>

   <para>
    The following functions are available to delay execution of the server
    process:
<synopsis>
pg_sleep ( <type>double precision</type> )
pg_sleep_for ( <type>interval</type> )
pg_sleep_until ( <type>timestamp with time zone</type> )
</synopsis>

    <function>pg_sleep</function> makes the current session's process
    sleep until the given number of seconds have
    elapsed.  Fractional-second delays can be specified.
    <function>pg_sleep_for</function> is a convenience function to
    allow the sleep time to be specified as an <type>interval</type>.
    <function>pg_sleep_until</function> is a convenience function for when
    a specific wake-up time is desired.
    For example:

<programlisting>
SELECT pg_sleep(1.5);
SELECT pg_sleep_for('5 minutes');
SELECT pg_sleep_until('tomorrow 03:00');
</programlisting>
   </para>

   <note>
     <para>
      The effective resolution of the sleep interval is platform-specific;
      0.01 seconds is a common value.  The sleep delay will be at least as long
      as specified. It might be longer depending on factors such as server load.
      In particular, <function>pg_sleep_until</function> is not guaranteed to
      wake up exactly at the specified time, but it will not wake up any earlier.
     </para>
   </note>

   <warning>
     <para>
      Make sure that your session does not hold more locks than necessary
      when calling <function>pg_sleep</function> or its variants.  Otherwise
      other sessions might have to wait for your sleeping process, slowing down
      the entire system.
     </para>
   </warning>
  </sect2>

 </sect1>