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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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 Vsnapshot" /><link rel="prev" href="datetime-posix-timezone-specs.html" title="B.5. POSIX Time Zone Specifications" /><link rel="next" href="datetime-julian-dates.html" title="B.7. Julian Dates" /></head><body id="docContent" class="container-fluid col-10"><div 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 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="datetime-julian-dates.html" title="B.7. Julian Dates">Next</a></td></tr></table><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 <a href="#DATETIME-UNITS-HISTORY" class="id_link">#</a></h2></div></div></div><a id="id-1.11.3.9.2" class="indexterm"></a><p>3   The SQL standard states that <span class="quote">“<span class="quote">Within the definition of a4   <span class="quote">‘<span class="quote">datetime literal</span>’</span>, the <span class="quote">‘<span class="quote">datetime5   values</span>’</span> are constrained by the natural rules for dates and6   times according to the Gregorian calendar</span>”</span>.7   <span class="productname">PostgreSQL</span> follows the SQL8   standard's lead by counting dates exclusively in the Gregorian9   calendar, even for years before that calendar was in use.10   This rule is known as the <em class="firstterm">proleptic Gregorian calendar</em>.11  </p><p>12   The Julian calendar was introduced by Julius Caesar in 45 BC.13   It was in common use in the Western world14   until the year 1582, when countries started changing to the Gregorian15   calendar.  In the Julian calendar, the tropical year is16   approximated as 365 1/4 days = 365.25 days. This gives an error of17   about 1 day in 128 years.18  </p><p>19   The accumulating calendar error prompted20   Pope Gregory XIII to reform the calendar in accordance with21   instructions from the Council of Trent.22   In the Gregorian calendar, the tropical year is approximated as23   365 + 97 / 400 days = 365.2425 days. Thus it takes approximately 330024   years for the tropical year to shift one day with respect to the25   Gregorian calendar.26  </p><p>27   The approximation 365+97/400 is achieved by having 97 leap years28   every 400 years, using the following rules:29 30   </p><table border="0" summary="Simple list" class="simplelist"><tr><td>31     Every year divisible by 4 is a leap year.32    </td></tr><tr><td>33     However, every year divisible by 100 is not a leap year.34    </td></tr><tr><td>35     However, every year divisible by 400 is a leap year after all.36    </td></tr></table><p>37 38   So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,39   2000, and 2400 are leap years.40 41   By contrast, in the older Julian calendar all years divisible by 4 are leap42   years.43  </p><p>44   The papal bull of February 1582 decreed that 10 days should be dropped45   from October 1582 so that 15 October should follow immediately after46   4 October.47   This was observed in Italy, Poland, Portugal, and Spain. Other Catholic48   countries followed shortly after, but Protestant countries were49   reluctant to change, and the Greek Orthodox countries didn't change50   until the start of the 20th century.51 52   The reform was observed by Great Britain and its dominions (including what53   is now the USA) in 1752.54   Thus 2 September 1752 was followed by 14 September 1752.55 56   This is why Unix systems that have the <code class="command">cal</code> program57   produce the following:58 59</p><pre class="screen">60$ <strong class="userinput"><code>cal 9 1752</code></strong>61   September 175262 S  M Tu  W Th  F  S63       1  2 14 15 166417 18 19 20 21 22 236524 25 26 27 28 29 3066</pre><p>67 68   But, of course, this calendar is only valid for Great Britain and69   dominions, not other places.70   Since it would be difficult and confusing to try to track the actual71   calendars that were in use in various places at various times,72   <span class="productname">PostgreSQL</span> does not try, but rather follows the Gregorian73   calendar rules for all dates, even though this method is not historically74   accurate.75  </p><p>76   Different calendars have been developed in various parts of the77   world, many predating the Gregorian system.78 79   For example,80   the beginnings of the Chinese calendar can be traced back to the 14th81   century BC. Legend has it that the Emperor Huangdi invented that82   calendar in 2637 BC.83 84   The People's Republic of China uses the Gregorian calendar85   for civil purposes. The Chinese calendar is used for determining86   festivals.87  </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" title="B.5. POSIX Time Zone Specifications">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="datetime-appendix.html" title="Appendix B. Date/Time Support">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="datetime-julian-dates.html" title="B.7. Julian Dates">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" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> B.7. Julian Dates</td></tr></table></div></body></html>