diff options
| -rw-r--r-- | doc/src/sgml/datetime.sgml | 46 | ||||
| -rw-r--r-- | doc/src/sgml/func.sgml | 22 |
2 files changed, 61 insertions, 7 deletions
diff --git a/doc/src/sgml/datetime.sgml b/doc/src/sgml/datetime.sgml index 2653d620c2e..d9666ebe7d9 100644 --- a/doc/src/sgml/datetime.sgml +++ b/doc/src/sgml/datetime.sgml @@ -785,9 +785,6 @@ <indexterm zone="datetime-units-history"> <primary>Gregorian calendar</primary> </indexterm> - <indexterm zone="datetime-units-history"> - <primary>Julian date</primary> - </indexterm> <para> The SQL standard states that <quote>Within the definition of a @@ -890,14 +887,27 @@ $ <userinput>cal 9 1752</userinput> festivals. </para> + </sect1> + + <sect1 id="datetime-julian-dates"> + <title>Julian Dates</title> + + <indexterm zone="datetime-julian-dates"> + <primary>Julian date</primary> + </indexterm> + <para> - The <firstterm>Julian Date</firstterm> system is another type of - calendar, unrelated to the Julian calendar though it is confusingly + The <firstterm>Julian Date</firstterm> system is a method for + numbering days. It is + unrelated to the Julian calendar, though it is confusingly named similarly to that calendar. The Julian Date system was invented by the French scholar Joseph Justus Scaliger (1540-1609) and probably takes its name from Scaliger's father, the Italian scholar Julius Caesar Scaliger (1484-1558). + </para> + + <para> In the Julian Date system, each day has a sequential number, starting from JD 0 (which is sometimes called <emphasis>the</> Julian Date). JD 0 corresponds to 1 January 4713 BC in the Julian calendar, or @@ -913,7 +923,31 @@ $ <userinput>cal 9 1752</userinput> input and output of dates (and also uses Julian dates for some internal datetime calculations), it does not observe the nicety of having dates run from noon to noon. <productname>PostgreSQL</> treats a Julian Date - as running from midnight to midnight. + as running from local midnight to local midnight, the same as a normal + date. + </para> + + <para> + This definition does, however, provide a way to obtain the astronomical + definition when you need it: do the arithmetic in time + zone <literal>UTC-12</literal>. For example, +<programlisting> +=> SELECT extract(julian from '2021-06-23 7:00:00-04'::timestamptz at time zone 'UTC-12'); + date_part +-------------------- + 2459389.9583333335 +(1 row) +=> SELECT extract(julian from '2021-06-23 8:00:00-04'::timestamptz at time zone 'UTC-12'); + date_part +----------- + 2459390 +(1 row) +=> SELECT extract(julian from date '2021-06-24'); + date_part +----------- + 2459390 +(1 row) +</programlisting> </para> </sect1> diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index fa9df4ad1d0..c51354d81df 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -6076,7 +6076,8 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}'); </row> <row> <entry><literal>J</literal></entry> - <entry>Julian Day (integer days since November 24, 4714 BC at midnight UTC)</entry> + <entry>Julian Date (integer days since November 24, 4714 BC at local + midnight; see <xref linkend="datetime-julian-dates">)</entry> </row> <row> <entry><literal>Q</literal></entry> @@ -7730,6 +7731,25 @@ SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-02'); </varlistentry> <varlistentry> + <term><literal>julian</literal></term> + <listitem> + <para> + The <firstterm>Julian Date</firstterm> corresponding to the + date or timestamp (not applicable to intervals). 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>2453737.5</computeroutput> +</screen> + </listitem> + </varlistentry> + + <varlistentry> <term><literal>microseconds</literal></term> <listitem> <para> |