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<appendix id="datetime-appendix">

<title>Date/Time Support</title>

<title>History of Units</title>

<para>

The Julian Date 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).

Astronomers have used the Julian period to assign a unique number to

every day since 1 January 4713 BC. This is the so-called Julian Date

(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC

to noon UTC on 2 January 4713 BC.

</para>

<para>

The <quote>Julian Date</quote> is different from the <quote>Julian

Calendar</quote>. The Julian calendar

was introduced by Julius Caesar in 45 BC. It was in common use

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

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 Greekorthodox countries didn't change

reluctant to change, and the GreekOrthodox countries didn't change

until the start of the 20th century.

The reform was observed by Great Britain and Dominions (including what is

</screen>

</para>

<note>

The SQL standard states that <quote>Within thedefinition of a

<quote>datetime literal</quote>,the<quote>datetime

value</quote>s are constrained by the natural rules for dates and

hence valid dates.

</note>

<para>

<quote>datetime literal</quote>, the<quote>datetime

value</quote>s are constrained bythenatural rules for dates and

times according to the Gregorian calendar</quote>. Dates between

standard's lead by counting dates exclusively in the Gregorian

</para>

<para>

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 inventedthe

century BC. Legend has it that the Emperor Huangdi inventedthat

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.

</para>

<para>

The <quote>Julian Date</quote> is unrelated to the <quote>Julian

calendar</quote>.

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 <emphasis>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

1 January 4713 BC to noon UTC on 2 January 4713 BC.

</para>

<para>

Although <productname>PostgreSQL</> supports Julian Date notation for

input and output of dates (and also uses them for some internal datetime

calculations), it does not observe the nicety of having dates run from

noon to noon. <productname>PostgreSQL</> treats a Julian Date as running

from midnight to midnight.

</para>

</sect1>

</appendix>

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<chapter id="functions">

<title>Functions and Operators</title>

</row>

<row>

<entry><literal>J</literal></entry>

<entry>Julian Day (days sinceJanuary 1, 4712 BC at midnight)</entry>

<entry>Julian Day (days sinceNovember 24, 4714 BC at midnight)</entry>

</row>

<row>

<entry><literal>Q</literal></entry>