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>B.6. 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
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>     Every year divisible by 4 is a leap year.
    </TD
></TR
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><TD
>     However, every year divisible by 100 is not a leap year.
    </TD
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><TD
>     However, every year divisible by 400 is a leap year after all.
    </TD
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></TBODY
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><P
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>

   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 that 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.
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