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>B.7. Julian Dates</A
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>   The <I
CLASS="FIRSTTERM"
>Julian Date</I
> 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).
  </P
><P
>   In the Julian Date system, each day has a sequential number, starting
   from JD 0 (which is sometimes called <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>the</I
></SPAN
> Julian Date).
   JD 0 corresponds to 1 January 4713 BC in the Julian calendar, or
   24 November 4714 BC in the Gregorian calendar.  Julian Date counting
   is most often used by astronomers for labeling their nightly observations,
   and therefore a date runs from noon UTC to the next noon UTC, rather than
   from midnight to midnight: JD 0 designates the 24 hours from noon UTC on
   24 November 4714 BC to noon UTC on 25 November 4714 BC.
  </P
><P
>   Although <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports Julian Date notation for
   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.  <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> treats a Julian Date
   as running from local midnight to local midnight, the same as a normal
   date.
  </P
><P
>   This definition does, however, provide a way to obtain the astronomical
   definition when you need it: do the arithmetic in time
   zone <TT
CLASS="LITERAL"
>UTC+12</TT
>.  For example,
</P><PRE
CLASS="PROGRAMLISTING"
>=&gt; SELECT extract(julian from '2021-06-23 7:00:00-04'::timestamptz at time zone 'UTC+12');
     date_part
--------------------
 2459388.9583333335
(1 row)
=&gt; SELECT extract(julian from '2021-06-23 8:00:00-04'::timestamptz at time zone 'UTC+12');
 date_part
-----------
   2459389
(1 row)
=&gt; SELECT extract(julian from date '2021-06-23');
 date_part
-----------
   2459389
(1 row)</PRE
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