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><A
NAME="DATETIME-UNITS-HISTORY"
>B.4. History of Units</A
></H1
><P
> The SQL standard states that <SPAN
CLASS="QUOTE"
>"Within the definition of a
<SPAN
CLASS="QUOTE"
>'datetime literal'</SPAN
>, the <SPAN
CLASS="QUOTE"
>'datetime
values'</SPAN
> are constrained by the natural rules for dates and
times according to the Gregorian calendar"</SPAN
>.
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> follows the SQL
standard's lead by counting dates exclusively in the Gregorian
calendar, even for years before that calendar was in use.
This rule is known as the <I
CLASS="FIRSTTERM"
>proleptic Gregorian calendar</I
>.
</P
><P
> The Julian calendar was introduced by Julius Caesar in 45 BC.
It was in common use in the Western world
until the year 1582, when countries started changing to the Gregorian
calendar. In the Julian calendar, the tropical year is
approximated as 365 1/4 days = 365.25 days. This gives an error of
about 1 day in 128 years.
</P
><P
> The accumulating calendar error prompted
Pope Gregory XIII to reform the calendar in accordance with
instructions from the Council of Trent.
In the Gregorian calendar, the tropical year is approximated as
365 + 97 / 400 days = 365.2425 days. Thus it takes approximately 3300
years for the tropical year to shift one day with respect to the
Gregorian calendar.
</P
><P
> The approximation 365+97/400 is achieved by having 97 leap years
every 400 years, using the following rules:
<P
></P
><TABLE
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><TD
> Every year divisible by 4 is a leap year.
</TD
></TR
><TR
><TD
> However, every year divisible by 100 is not a leap year.
</TD
></TR
><TR
><TD
> However, every year divisible by 400 is a leap year after all.
</TD
></TR
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><P
></P
>
So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,
2000, and 2400 are leap years.
By contrast, in the older Julian calendar all years divisible by 4 are leap
years.
</P
><P
> The papal bull of February 1582 decreed that 10 days should be dropped
from October 1582 so that 15 October should follow immediately after
4 October.
This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
countries followed shortly after, but Protestant countries were
reluctant to change, and the Greek Orthodox countries didn't change
until the start of the 20th century.
The reform was observed by Great Britain and its dominions (including what
is now the USA) in 1752.
Thus 2 September 1752 was followed by 14 September 1752.
This is why Unix systems have the <TT
CLASS="COMMAND"
>cal</TT
> program
produce the following:
</P><PRE
CLASS="SCREEN"
>$ <KBD
CLASS="USERINPUT"
>cal 9 1752</KBD
>
September 1752
S M Tu W Th F S
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30</PRE
><P>
But, of course, this calendar is only valid for Great Britain and
dominions, not other places.
Since it would be difficult and confusing to try to track the actual
calendars that were in use in various places at various times,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does not try, but rather follows the Gregorian
calendar rules for all dates, even though this method is not historically
accurate.
</P
><P
> Different calendars have been developed in various parts of the
world, many predating the Gregorian system.
For example,
the beginnings of the Chinese calendar can be traced back to the 14th
century BC. Legend has it that the Emperor Huangdi invented that
calendar in 2637 BC.
The People's Republic of China uses the Gregorian calendar
for civil purposes. The Chinese calendar is used for determining
festivals.
</P
><P
> The <I
CLASS="FIRSTTERM"
>Julian Date</I
> system is another type of
calendar, 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 <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 midnight to midnight.
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