describes
   the current (as of 2020) timekeeping practice in Paris.  This specification
   says that standard time has the abbreviation <literal>CET</literal> and
   is one hour ahead (east) of UTC; daylight savings time has the
   abbreviation <literal>CEST</literal> and is implicitly two hours ahead
   of UTC; daylight savings time begins on the last Sunday in March at 2AM
   CET and ends on the last Sunday in October at 3AM CEST.
  </para>

  <para>
   The four timezone names <literal>EST5EDT</literal>,
   <literal>CST6CDT</literal>, <literal>MST7MDT</literal>,
   and <literal>PST8PDT</literal> look like they are POSIX zone
   specifications.  However, they actually are treated as named time zones
   because (for historical reasons) there are files by those names in the
   IANA time zone database.  The practical implication of this is that
   these zone names will produce valid historical USA daylight-savings
   transitions, even when a plain POSIX specification would not.
  </para>

  <para>
   One should be wary that it is easy to misspell a POSIX-style time zone
   specification, since there is no check on the reasonableness of the
   zone abbreviation(s).  For example, <literal>SET TIMEZONE TO
   FOOBAR0</literal> will work, leaving the system effectively using a
   rather peculiar abbreviation for UTC.
  </para>

  </sect1>

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

  <indexterm zone="datetime-units-history">
   <primary>Gregorian calendar</primary>
  </indexterm>

  <para>
   The SQL standard states that <quote>Within the definition of a
   <quote>datetime literal</quote>, the <quote>datetime
   values</quote> are constrained by the natural rules for dates and
   times according to the Gregorian calendar</quote>.
   <productname>PostgreSQL</productname> 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 <firstterm>proleptic Gregorian calendar</firstterm>.
  </para>

  <para>
   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.
  </para>

  <para>
   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.
  </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 all 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,