B.7. Julian Dates

B.7. Julian Dates
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+ The Julian Date 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). +

+ In the Julian Date system, each day has a sequential number, starting + from JD 0 (which is sometimes called the 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. +

+ Although PostgreSQL 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. PostgreSQL treats a Julian Date + as running from local midnight to local midnight, the same as a normal + date. +

+ This definition does, however, provide a way to obtain the astronomical + definition when you need it: do the arithmetic in time + zone UTC+12. For example, +

+=> SELECT extract(julian from '2021-06-23 7:00:00-04'::timestamptz at time zone 'UTC+12');
+           extract
+------------------------------
+ 2459388.95833333333333333333
+(1 row)
+=> SELECT extract(julian from '2021-06-23 8:00:00-04'::timestamptz at time zone 'UTC+12');
+               extract
+--------------------------------------
+ 2459389.0000000000000000000000000000
+(1 row)
+=> SELECT extract(julian from date '2021-06-23');
+ extract
+---------
+ 2459389
+(1 row)
+

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