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- <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>B.6. History of Units</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="datetime-posix-timezone-specs.html" title="B.5. POSIX Time Zone Specifications" /><link rel="next" href="sql-keywords-appendix.html" title="Appendix C. SQL Key Words" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">B.6. History of Units</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="datetime-posix-timezone-specs.html" title="B.5. POSIX Time Zone Specifications">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="datetime-appendix.html" title="Appendix B. Date/Time Support">Up</a></td><th width="60%" align="center">Appendix B. Date/Time Support</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-keywords-appendix.html" title="Appendix C. SQL Key Words">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="DATETIME-UNITS-HISTORY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">B.6. History of Units</h2></div></div></div><a id="id-1.11.3.9.2" class="indexterm"></a><a id="id-1.11.3.9.3" class="indexterm"></a><p>
- The SQL standard states that <span class="quote">“<span class="quote">Within the definition of a
- <span class="quote">‘<span class="quote">datetime literal</span>’</span>, the <span class="quote">‘<span class="quote">datetime
- values</span>’</span> are constrained by the natural rules for dates and
- times according to the Gregorian calendar</span>”</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 <em class="firstterm">proleptic Gregorian calendar</em>.
- </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><table border="0" summary="Simple list" class="simplelist"><tr><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></table><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 that have the <code class="command">cal</code> program
- produce the following:
-
- </p><pre class="screen">
- $ <strong class="userinput"><code>cal 9 1752</code></strong>
- 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 <em class="firstterm">Julian Date</em> 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"><em>the</em></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.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="datetime-posix-timezone-specs.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="datetime-appendix.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-keywords-appendix.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">B.5. <acronym class="acronym">POSIX</acronym> Time Zone Specifications </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Appendix C. <acronym class="acronym">SQL</acronym> Key Words</td></tr></table></div></body></html>
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