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Diffstat (limited to 'doc/src/sgml/datetime.sgml')
| -rw-r--r-- | doc/src/sgml/datetime.sgml | 46 |
1 files changed, 40 insertions, 6 deletions
diff --git a/doc/src/sgml/datetime.sgml b/doc/src/sgml/datetime.sgml index 39fbc39cb0d..2e48d5d7885 100644 --- a/doc/src/sgml/datetime.sgml +++ b/doc/src/sgml/datetime.sgml @@ -763,9 +763,6 @@ <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 @@ -868,14 +865,27 @@ $ <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</emphasis> Julian Date). JD 0 corresponds to 1 January 4713 BC in the Julian calendar, or @@ -891,7 +901,31 @@ $ <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</productname> 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'); + extract +------------------------------ + 2459389.95833333333333333333 +(1 row) +=> SELECT extract(julian from '2021-06-23 8:00:00-04'::timestamptz at time zone 'UTC-12'); + extract +-------------------------------------- + 2459390.0000000000000000000000000000 +(1 row) +=> SELECT extract(julian from date '2021-06-24'); + extract +--------- + 2459390 +(1 row) +</programlisting> </para> </sect1> |