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$Header: /cvsroot/pgsql/doc/src/sgml/datetime.sgml,v 2.25 2002/01/04 17:02:25 thomas Exp $
Date/time details
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 <appendix id="datetime-appendix">
  <title id="datetime-appendix-title">Date/Time Support</title>

  <para>
   <productname>PostgreSQL</productname> uses an internal heuristic
   parser for all date/time support. Dates and times are input as
   strings, and are broken up into distinct fields with a preliminary
   determination of what kind of information may be in the
   field. Each field is interpreted and either assigned a numeric
   value, ignored, or rejected.
   The parser contains internal lookup tables for all textual fields,
   including months, days of the week, and time
   zones.
  </para>

  <para>
   This appendix includes information on the content of these
   lookup tables and describes the steps used by the parser to decode
   dates and times.
  </para>

  <sect1>
   <title>Date/Time Keywords</title>

   <para>
    <table tocentry="1">
     <title>Month Abbreviations</title>
     <tgroup cols="2">
      <thead>
       <row>
	<entry>Month</entry>
	<entry>Abbreviations</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>April</entry>
	<entry>Apr</entry>
       </row>
       <row>
	<entry>August</entry>
	<entry>Aug</entry>
       </row>
       <row>
	<entry>December</entry>
	<entry>Dec</entry>
       </row>
       <row>
	<entry>February</entry>
	<entry>Feb</entry>
       </row>
       <row>
	<entry>January</entry>
	<entry>Jan</entry>
       </row>
       <row>
	<entry>July</entry>
	<entry>Jul</entry>
       </row>
       <row>
	<entry>June</entry>
	<entry>Jun</entry>
       </row>
       <row>
	<entry>March</entry>
	<entry>Mar</entry>
       </row>
       <row>
	<entry>November</entry>
	<entry>Nov</entry>
       </row>
       <row>
	<entry>October</entry>
	<entry>Oct</entry>
       </row>
       <row>
	<entry>September</entry>
	<entry>Sep, Sept</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <note>
     <para>
      The month <literal>May</literal> has no explicit abbreviation, for obvious reasons.
     </para>
    </note>
   </para>

    <para>
     <table tocentry="1">
      <title>Day of the Week Abbreviations</title>
      <tgroup cols="2">
       <thead>
	<row>
	 <entry>Day</entry>
	 <entry>Abbreviation</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>Sunday</entry>
	 <entry>Sun</entry>
	</row>
	<row>
	 <entry>Monday</entry>
	 <entry>Mon</entry>
	</row>
	<row>
	 <entry>Tuesday</entry>
	 <entry>Tue, Tues</entry>
	</row>
	<row>
	 <entry>Wednesday</entry>
	 <entry>Wed, Weds</entry>
	</row>
	<row>
	 <entry>Thursday</entry>
	 <entry>Thu, Thur, Thurs</entry>
	</row>
	<row>
	 <entry>Friday</entry>
	 <entry>Fri</entry>
	</row>
	<row>
	 <entry>Saturday</entry>
	 <entry>Sat</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>

   <para>
    <table tocentry="1">
     <title><productname>PostgreSQL</productname> Field Modifiers</title>
     <titleabbrev>Field Modifiers</titleabbrev>
     <tgroup cols="2">
      <thead>
       <row>
	<entry>Identifier</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>ABSTIME</entry>
	<entry>Keyword ignored</entry>
       </row>
       <row>
	<entry>AM</entry>
	<entry>Time is before 12:00</entry>
       </row>
       <row>
	<entry>AT</entry>
	<entry>Keyword ignored</entry>
       </row>
       <row>
	<entry>JULIAN, JD, J</entry>
	<entry>Next field is Julian Day</entry>
       </row>
       <row>
	<entry>ON</entry>
	<entry>Keyword ignored</entry>
       </row>
       <row>
	<entry>PM</entry>
	<entry>Time is on or after after 12:00</entry>
       </row>
       <row>
	<entry>T</entry>
	<entry>Next field is time</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <para>
    The keyword <literal>ABSTIME</literal> is ignored for historical
    reasons; in very old releases of
    <productname>PostgreSQL</productname> invalid <type>ABSTIME</type>
    fields were emitted as <quote>Invalid Abstime</quote>. This is no
    longer the case however and this keyword will likely be dropped in
    a future release.
   </para>
  </sect1>

  <sect1 id="timezones">
   <title>Time Zones</title>

   <indexterm zone="timezones">
    <primary>time zones</primary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> contains internal tabular
    information for time zone decoding, since there is no *nix standard
    system interface to provide access to general, cross-timezone
    information. The underlying OS <emphasis>is</emphasis> used to
    provide time zone information for <emphasis>output</emphasis>, however.
   </para>

   <para>
    The following table of time zones recognized by
    <productname>PostgreSQL</productname> is organized by time
    zone offset from UTC, rather than alphabetically; this is intended
    to faciliate
    matching local usage with recognized abreviations for cases where
    these might differ.

    <table tocentry="1">
     <title><productname>PostgreSQL</productname> Recognized Time Zones</title>
     <titleabbrev>Time Zones</titleabbrev>
     <tgroup cols="3">
      <thead>
       <row>
	<entry>Time Zone</entry>
	<entry>Offset from UTC</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>NZDT</entry>
	<entry>+13:00</entry>
	<entry>New Zealand Daylight Time</entry>
       </row>
       <row>
	<entry>IDLE</entry>
	<entry>+12:00</entry>
	<entry>International Date Line, East</entry>
       </row>
       <row>
	<entry>NZST</entry>
	<entry>+12:00</entry>
	<entry>New Zealand Standard Time</entry>
       </row>
       <row>
	<entry>NZT</entry>
	<entry>+12:00</entry>
	<entry>New Zealand Time</entry>
       </row>
       <row>
	<entry>AESST</entry>
	<entry>+11:00</entry>
	<entry>Australia Eastern Summer Standard Time</entry>
       </row>
       <row>
	<entry>ACSST</entry>
	<entry>+10:30</entry>
	<entry>Central Australia Summer Standard Time</entry>
       </row>
       <row>
	<entry>CADT</entry>
	<entry>+10:30</entry>
	<entry>Central Australia Daylight Savings Time</entry>
       </row>
       <row>
	<entry>SADT</entry>
	<entry>+10:30</entry>
	<entry>South Australian Daylight Time</entry>
       </row>
       <row>
	<entry>AEST</entry>
	<entry>+10:00</entry>
	<entry>Australia Eastern Standard Time</entry>
       </row>
       <row>
	<entry>EAST</entry>
	<entry>+10:00</entry>
	<entry>East Australian Standard Time</entry>
       </row>
       <row>
	<entry>GST</entry>
	<entry>+10:00</entry>
	<entry>Guam Standard Time, USSR Zone 9</entry>
       </row>
       <row>
	<entry>LIGT</entry>
	<entry>+10:00</entry>
	<entry>Melbourne, Australia</entry>
       </row>
       <row>
	<entry>SAST</entry>
	<entry>+09:30</entry>
	<entry>South Australia Standard Time</entry>
       </row>
       <row>
	<entry>CAST</entry>
	<entry>+09:30</entry>
	<entry>Central Australia Standard Time</entry>
       </row>
       <row>
	<entry>AWSST</entry>
	<entry>+09:00</entry>
	<entry>Australia Western Summer Standard Time</entry>
       </row>
       <row>
	<entry>JST</entry>
	<entry>+09:00</entry>
	<entry>Japan Standard Time,USSR Zone 8</entry>
       </row>
       <row>
	<entry>KST</entry>
	<entry>+09:00</entry>
	<entry>Korea Standard Time</entry>
       </row>
       <row>
	<entry>MHT</entry>
	<entry>+09:00</entry>
	<entry>Kwajalein Time</entry>
       </row>
       <row>
	<entry>WDT</entry>
	<entry>+09:00</entry>
	<entry>West Australian Daylight Time</entry>
       </row>
       <row>
	<entry>MT</entry>
	<entry>+08:30</entry>
	<entry>Moluccas Time</entry>
       </row>
       <row>
	<entry>AWST</entry>
	<entry>+08:00</entry>
	<entry>Australia Western Standard Time</entry>
       </row>
       <row>
	<entry>CCT</entry>
	<entry>+08:00</entry>
	<entry>China Coastal Time</entry>
       </row>
       <row>
	<entry>WADT</entry>
	<entry>+08:00</entry>
	<entry>West Australian Daylight Time</entry>
       </row>
       <row>
	<entry>WST</entry>
	<entry>+08:00</entry>
	<entry>West Australian Standard Time</entry>
       </row>
       <row>
	<entry>JT</entry>
	<entry>+07:30</entry>
	<entry>Java Time</entry>
       </row>
       <row>
	<entry>ALMST</entry>
	<entry>+07:00</entry>
	<entry>Almaty Summer Time</entry>
       </row>
       <row>
	<entry>WAST</entry>
	<entry>+07:00</entry>
	<entry>West Australian Standard Time</entry>
       </row>
       <row>
	<entry>CXT</entry>
	<entry>+07:00</entry>
	<entry>Christmas (Island) Time</entry>
       </row>
       <row>
	<entry>ALMT</entry>
	<entry>+06:00</entry>
	<entry>Almaty Time</entry>
       </row>
<!--
       Conflicts with China Coastal Time
       <row>
	<entry>CCT</entry>
	<entry>+06:30</entry>
	<entry>Cocos Island Time</entry>
       </row>
-->
       <row>
	<entry>MAWT</entry>
	<entry>+06:00</entry>
	<entry>Mawson (Antarctica) Time</entry>
       </row>
       <row>
	<entry>IOT</entry>
	<entry>+05:00</entry>
	<entry>Indian Chagos Time</entry>
       </row>
       <row>
	<entry>MVT</entry>
	<entry>+05:00</entry>
	<entry>Maldives Island Time</entry>
       </row>
       <row>
	<entry>TFT</entry>
	<entry>+05:00</entry>
	<entry>Kerguelen Time</entry>
       </row>
       <row>
	<entry>AFT</entry>
	<entry>+04:30</entry>
	<entry>Afganistan Time</entry>
       </row>
       <row>
	<entry>EAST</entry>
	<entry>+04:00</entry>
	<entry>Antananarivo Savings Time</entry>
       </row>
       <row>
	<entry>MUT</entry>
	<entry>+04:00</entry>
	<entry>Mauritius Island Time</entry>
       </row>
       <row>
	<entry>RET</entry>
	<entry>+04:00</entry>
	<entry>Reunion Island Time</entry>
       </row>
       <row>
	<entry>SCT</entry>
	<entry>+04:00</entry>
	<entry>Mahe Island Time</entry>
       </row>
       <row>
	<entry>IT</entry>
	<entry>+03:30</entry>
	<entry>Iran Time</entry>
       </row>
       <row>
	<entry>EAT</entry>
	<entry>+03:00</entry>
	<entry>Antananarivo, Comoro Time</entry>
       </row>
       <row>
	<entry>BT</entry>
	<entry>+03:00</entry>
	<entry>Baghdad Time</entry>
       </row>
       <row>
	<entry>EETDST</entry>
	<entry>+03:00</entry>
	<entry>Eastern Europe Daylight Savings Time</entry>
       </row>
       <row>
	<entry>HMT</entry>
	<entry>+03:00</entry>
	<entry>Hellas Mediterranean Time (?)</entry>
       </row>
       <row>
	<entry>BDST</entry>
	<entry>+02:00</entry>
	<entry>British Double Standard Time</entry>
       </row>
       <row>
	<entry>CEST</entry>
	<entry>+02:00</entry>
	<entry>Central European Savings Time</entry>
       </row>
       <row>
	<entry>CETDST</entry>
	<entry>+02:00</entry>
	<entry>Central European Daylight Savings Time</entry>
       </row>
       <row>
	<entry>EET</entry>
	<entry>+02:00</entry>
	<entry>Eastern Europe, USSR Zone 1</entry>
       </row>
       <row>
	<entry>FWT</entry>
	<entry>+02:00</entry>
	<entry>French Winter Time</entry>
       </row>
       <row>
	<entry>IST</entry>
	<entry>+02:00</entry>
	<entry>Israel Standard Time</entry>
       </row>
       <row>
	<entry>MEST</entry>
	<entry>+02:00</entry>
	<entry>Middle Europe Summer Time</entry>
       </row>
       <row>
	<entry>METDST</entry>
	<entry>+02:00</entry>
	<entry>Middle Europe Daylight Time</entry>
       </row>
       <row>
	<entry>SST</entry>
	<entry>+02:00</entry>
	<entry>Swedish Summer Time</entry>
       </row>
       <row>
	<entry>BST</entry>
	<entry>+01:00</entry>
	<entry>British Summer Time</entry>
       </row>
       <row>
	<entry>CET</entry>
	<entry>+01:00</entry>
	<entry>Central European Time</entry>
       </row>
       <row>
	<entry>DNT</entry>
	<entry>+01:00</entry>
	<entry><foreignphrase>Dansk Normal Tid</foreignphrase></entry>
       </row>
       <row>
	<entry>FST</entry>
	<entry>+01:00</entry>
	<entry>French Summer Time</entry>
       </row>
       <row>
	<entry>MET</entry>
	<entry>+01:00</entry>
	<entry>Middle Europe Time</entry>
       </row>
       <row>
	<entry>MEWT</entry>
	<entry>+01:00</entry>
	<entry>Middle Europe Winter Time</entry>
       </row>
       <row>
	<entry>MEZ</entry>
	<entry>+01:00</entry>
	<entry>Middle Europe Zone</entry>
       </row>
       <row>
	<entry>NOR</entry>
	<entry>+01:00</entry>
	<entry>Norway Standard Time</entry>
       </row>
       <row>
	<entry>SET</entry>
	<entry>+01:00</entry>
	<entry>Seychelles Time</entry>
       </row>
       <row>
	<entry>SWT</entry>
	<entry>+01:00</entry>
	<entry>Swedish Winter Time</entry>
       </row>
       <row>
	<entry>WETDST</entry>
	<entry>+01:00</entry>
	<entry>Western Europe Daylight Savings Time</entry>
       </row>
       <row>
	<entry>GMT</entry>
	<entry>+00:00</entry>
	<entry>Greenwich Mean Time</entry>
       </row>
       <row>
	<entry>UT</entry>
	<entry>+00:00</entry>
	<entry>Universal Time</entry>
       </row>
       <row>
	<entry>UTC</entry>
	<entry>+00:00</entry>
	<entry>Universal Time, Coordinated</entry>
       </row>
       <row>
	<entry>Z</entry>
	<entry>+00:00</entry>
	<entry>Same as UTC</entry>
       </row>
       <row>
	<entry>ZULU</entry>
	<entry>+00:00</entry>
	<entry>Same as UTC</entry>
       </row>
       <row>
	<entry>WET</entry>
	<entry>+00:00</entry>
	<entry>Western Europe</entry>
       </row>
       <row>
	<entry>WAT</entry>
	<entry>-01:00</entry>
	<entry>West Africa Time</entry>
       </row>
       <row>
	<entry>NDT</entry>
	<entry>-02:30</entry>
	<entry>Newfoundland Daylight Time</entry>
       </row>
       <row>
	<entry>ADT</entry>
	<entry>-03:00</entry>
	<entry>Atlantic Daylight Time</entry>
       </row>
       <row>
	<entry>AWT</entry>
	<entry>-03:00</entry>
	<entry>(unknown)</entry>
       </row>
       <row>
	<entry>NFT</entry>
	<entry>-03:30</entry>
	<entry>Newfoundland Standard Time</entry>
       </row>
       <row>
	<entry>NST</entry>
	<entry>-03:30</entry>
	<entry>Newfoundland Standard Time</entry>
       </row>
       <row>
	<entry>AST</entry>
	<entry>-04:00</entry>
	<entry>Atlantic Standard Time (Canada)</entry>
       </row>
       <row>
	<entry>ACST</entry>
	<entry>-04:00</entry>
	<entry>Atlantic/Porto Acre Summer Time</entry>
       </row>
       <row>
	<entry>ACT</entry>
	<entry>-05:00</entry>
	<entry>Atlantic/Porto Acre Standard Time</entry>
       </row>
       <row>
	<entry>EDT</entry>
	<entry>-04:00</entry>
	<entry>Eastern Daylight Time</entry>
       </row>
       <!--
      <row>
      <entry>ZP4</entry>
      <entry>-04:00</entry>
      <entry>GMT +4 hours</entry>
      </row>
       -->
       <row>
	<entry>CDT</entry>
	<entry>-05:00</entry>
	<entry>Central Daylight Time</entry>
       </row>
       <row>
	<entry>EST</entry>
	<entry>-05:00</entry>
	<entry>Eastern Standard Time</entry>
       </row>
       <!--
      <row>
      <entry>ZP5</entry>
      <entry>-05:00</entry>
      <entry>GMT +5  hours</entry>
      </row>
       -->
       <row>
	<entry>CST</entry>
	<entry>-06:00</entry>
	<entry>Central Standard Time</entry>
       </row>
       <row>
	<entry>MDT</entry>
	<entry>-06:00</entry>
	<entry>Mountain Daylight Time</entry>
       </row>
       <!--
      <row>
      <entry>ZP6</entry>
      <entry>-06:00</entry>
      <entry>GMT +6  hours</entry>
      </row>
       -->
       <row>
	<entry>MST</entry>
	<entry>-07:00</entry>
	<entry>Mountain Standard Time</entry>
       </row>
       <row>
	<entry>PDT</entry>
	<entry>-07:00</entry>
	<entry>Pacific Daylight Time</entry>
       </row>
       <row>
	<entry>AKDT</entry>
	<entry>-08:00</entry>
	<entry>Alaska Daylight Time</entry>
       </row>
       <row>
	<entry>PST</entry>
	<entry>-08:00</entry>
	<entry>Pacific Standard Time</entry>
       </row>
       <row>
	<entry>YDT</entry>
	<entry>-08:00</entry>
	<entry>Yukon Daylight Time</entry>
       </row>
       <row>
	<entry>AKST</entry>
	<entry>-09:00</entry>
	<entry>Alaska Standard Time</entry>
       </row>
       <row>
	<entry>HDT</entry>
	<entry>-09:00</entry>
	<entry>Hawaii/Alaska Daylight Time</entry>
       </row>
       <row>
	<entry>YST</entry>
	<entry>-09:00</entry>
	<entry>Yukon Standard Time</entry>
       </row>
       <row>
	<entry>AHST</entry>
	<entry>-10:00</entry>
	<entry>Alaska-Hawaii Standard Time</entry>
       </row>
       <row>
	<entry>HST</entry>
	<entry>-10:00</entry>
	<entry>Hawaii Standard Time</entry>
       </row>
       <row>
	<entry>CAT</entry>
	<entry>-10:00</entry>
	<entry>Central Alaska Time</entry>
       </row>
       <row>
	<entry>NT</entry>
	<entry>-11:00</entry>
	<entry>Nome Time</entry>
       </row>
       <row>
	<entry>IDLW</entry>
	<entry>-12:00</entry>
	<entry>International Date Line, West</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

  <sect2>
   <title>Australian Time Zones</title>

   <para>
    Australian time zones and their naming variants
    account for fully one quarter of all time zones in the 
    <productname>PostgreSQL</productname> time zone lookup table.
    There are two naming conflicts with time zones commonly used
    in the United States, <literal>CST</literal> and <literal>EST</literal>.
   </para>

   <para>
    If the runtime option <literal>AUSTRALIAN_TIMEZONES</literal> is set 
    then <literal>CST</literal>, <literal>EST</literal>, and
    <literal>SAT</literal> will be
    interpreted as Australian timezone names.  Without this option,
    <literal>CST</literal> and <literal>EST</literal> are taken as
    American timezone names, while <literal>SAT</literal> is interpreted as a
    noise word indicating <literal>Saturday</literal>.

    <table tocentry="1">
     <title><productname>PostgreSQL</productname> Australian Time Zones</title>
     <titleabbrev>Australian Time Zones</titleabbrev>
     <tgroup cols="3">
      <thead>
       <row>
	<entry>Time Zone</entry>
	<entry>Offset from UTC</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>ACST</entry>
	<entry>+09:30</entry>
	<entry>Central Australia Standard Time</entry>
       </row>
       <row>
	<entry>CST</entry>
	<entry>+10:30</entry>
	<entry>Australian Central Standard Time</entry>
       </row>
       <row>
	<entry>EST</entry>
	<entry>+10:00</entry>
	<entry>Australian Eastern Standard Time</entry>
       </row>
       <row>
	<entry>SAT</entry>
	<entry>+09:30</entry>
	<entry>South Australian Standard Time</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>
  </sect2>

  <sect2>
   <title>Date/Time Input Interpretation</title>

   <para>
    The date/time types are all decoded using a common set of routines.
   </para>

   <procedure>
    <title>Date/Time Input Interpretation</title>

    <step>
     <para>
      Break the input string into tokens and categorize each token as
      a string, time, time zone, or number.
     </para>

     <substeps>
      <step>
       <para>
	If the numeric token contains a colon (":"), this is a time
	 string. Include all subsequent digits and colons.
       </para>
      </step>

      <step>
       <para>
	If the numeric token contains a dash ("-"), slash ("/"), or
	 two or more dots ("."),
	this is a date string which may have a text month.
       </para>
      </step>

      <step>
       <para>
	If the token is numeric only, then it is either a single field
	or an ISO-8601 concatenated date
	 (e.g. <literal>19990113</literal> for January 13, 1999)
	or time (e.g. 141516 for 14:15:16).
       </para>
      </step>
      <step>
       <para>
	If the token starts with a plus ("+") or minus ("-"),
	then it is either a time zone or a special field.
       </para>
      </step>
     </substeps>
    </step>

    <step>
     <para>
      If the token is a text string, match up with possible strings.
     </para>
     
     <substeps>
      <step>
       <para>
	Do a binary-search table lookup for the token
	as either a special string (e.g. <literal>today</literal>),
	day (e.g. <literal>Thursday</literal>),
	month (e.g. <literal>January</literal>),
	or noise word (e.g. <literal>at</literal>, <literal>on</literal>).
       </para>
       <para>
	Set field values and bit mask for fields.
	For example, set year, month, day for <literal>today</literal>, 
	and additionally hour, minute, second for <literal>now</literal>.
       </para>
      </step>
      
      <step>
       <para>
	If not found, do a similar binary-search table lookup to match
	the token with a time zone.
       </para>
      </step>

      <step>
       <para>
	If not found, throw an error.
       </para>
      </step>
     </substeps>
    </step>
    
    <step>
     <para>
      The token is a number or number field.
     </para>

     <substeps>
      <step>
       <para>
	If there are more than 4 digits, 
	and if no other date fields have been previously read, then interpret 
	as a <quote>concatenated date</quote> (e.g. <literal>19990118</literal>). 8
	and 6 digits are interpreted as year, month, and day, while 7
	and 5 digits are interpreted as year, day of year, respectively.
       </para>
      </step>

      <step>
       <para>
	If the token is three digits
	and a year has already been decoded, then interpret as day of year.
       </para>
      </step>
      
      <step>
       <para>
	If four or six digits and a year has already been read, then
	 interpret as a time. 
       </para>
      </step>

      <step>
       <para>
	If four or more digits, then interpret as a year.
       </para>
      </step>

      <step>
       <para>
	If in European date mode, and if the day field has not yet been read,
	and if the value is less than or equal to 31, then interpret as a day.
       </para>
      </step>

      <step>
       <para>
	If the month field has not yet been read,
	and if the value is less than or equal to 12, then interpret as a month.
       </para>
      </step>

      <step>
       <para>
	If the day field has not yet been read,
	and if the value is less than or equal to 31, then interpret as a day.
       </para>
      </step>

      <step>
       <para>
	If two digits or four or more digits, then interpret as a year.
       </para>
      </step>

      <step>
       <para>
	Otherwise, throw an error.
       </para>
      </step>
     </substeps>
    </step>

    <step>
     <para>
      If BC has been specified, negate the year and add one for
      internal storage
      (there is no year zero in the Gregorian calendar, so numerically
      <literal>1BC</literal> becomes year zero).
     </para>
    </step>

    <step>
     <para>
      If BC was not specified, and if the year field was two digits in length, then
      adjust the year to 4 digits. If the field was less than 70, then add 2000;
      otherwise, add 1900.

      <tip>
       <para>
	Gregorian years 1-99AD may be entered by using 4 digits with leading
	zeros (e.g. 0099 is 99AD). Previous versions of
	<productname>PostgreSQL</productname> accepted years with three
	digits and with single digits, but as of version 7.0 the rules have
	been tightened up to reduce the possibility of ambiguity.
       </para>
      </tip>
     </para>
    </step>
   </procedure>
  </sect2>
 </sect1>

 <sect1 id="units-history">
  <title>History of Units</title>

  <note>
   <para>
    Contributed by José Soares (<email>jose@sferacarta.com</email>)
   </para>
  </note>

  <para>
   The Julian Day was invented by the French scholar
   Joseph Justus Scaliger (1540-1609)
   and probably takes its name from the 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 Day
   (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>
   <quote>Julian Day</quote> is different from <quote>Julian Date</quote>.

   The Julian calendar was introduced by Julius Caesar in 45 BC. It was
   in common use until the 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.
   The accumulating calendar error prompted  Pope Gregory XIII 
   to reform the calendar in accordance with instructions
   from the Council of Trent.
  </para>

  <para>   
   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.
  </para>

  <para>
   The approximation 365+97/400 is achieved by having 97 leap years
   every 400 years, using the following rules:

   <simplelist>
    <member>
     Every year divisible by 4 is a leap year.
    </member>
    <member>
     However, every year divisible by 100 is not a leap year.
    </member>
    <member>
     However, every year divisible by 400 is a leap year after all.
    </member>
   </simplelist>

   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 only years divisible by 4 are leap years.
  </para>

  <para>
   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 this century.

   The reform was observed by Great Britain and Dominions (including what is
   now the USA) in 1752.
   Thus 2 Sep 1752 was followed by 14 Sep 1752.

   This is why Unix systems have <application>cal</application>
   produce the following:

   <programlisting>
% cal 9 1752
   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
   </programlisting>
  </para>

  <note>
   <para>
    SQL92 states that
    <quote>Within the definition of a <sgmltag>datetime literal</sgmltag>,
     the <sgmltag>datetime value</sgmltag>s are constrained by the
     natural rules for dates and times
     according to the Gregorian calendar</quote>.
    Dates between 1752-09-03 and 1752-09-13, although eliminated in
    some countries by Papal fiat, conform to
    <quote>natural rules</quote> and are hence valid dates.
   </para>
  </note>

  <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 invented the
   calendar in 2637 BC.
   
   The People's Republic of China uses the Gregorian calendar
   for civil purposes. Chinese calendar is used for determining
   festivals.
  </para>
 </sect1>
</appendix>

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