<replaceable>offset</replaceable>
<replaceable>zone_abbreviation</replaceable> <replaceable>offset</replaceable> D
<replaceable>zone_abbreviation</replaceable> <replaceable>time_zone_name</replaceable>
@INCLUDE <replaceable>file_name</replaceable>
@OVERRIDE
</synopsis>
   </para>

   <para>
    A <replaceable>zone_abbreviation</replaceable> is just the abbreviation
    being defined.  An <replaceable>offset</replaceable> is an integer giving
    the equivalent offset in seconds from UTC, positive being east from
    Greenwich and negative being west.  For example, -18000 would be five
    hours west of Greenwich, or North American east coast standard time.
    <literal>D</literal> indicates that the zone name represents local
    daylight-savings time rather than standard time.
   </para>

   <para>
    Alternatively, a <replaceable>time_zone_name</replaceable> can be given, referencing
    a zone name defined in the IANA timezone database.  The zone's definition
    is consulted to see whether the abbreviation is or has been in use in
    that zone, and if so, the appropriate meaning is used &mdash; that is,
    the meaning that was currently in use at the timestamp whose value is
    being determined, or the meaning in use immediately before that if it
    wasn't current at that time, or the oldest meaning if it was used only
    after that time.  This behavior is essential for dealing with
    abbreviations whose meaning has historically varied.  It is also allowed
    to define an abbreviation in terms of a zone name in which that
    abbreviation does not appear; then using the abbreviation is just
    equivalent to writing out the zone name.
   </para>

   <tip>
    <para>
     Using a simple integer <replaceable>offset</replaceable> is preferred
     when defining an abbreviation whose offset from UTC has never changed,
     as such abbreviations are much cheaper to process than those that
     require consulting a time zone definition.
    </para>
   </tip>

   <para>
    The <literal>@INCLUDE</literal> syntax allows inclusion of another file in the
    <filename>.../share/timezonesets/</filename> directory.  Inclusion can be nested,
    to a limited depth.
   </para>

   <para>
    The <literal>@OVERRIDE</literal> syntax indicates that subsequent entries in the
    file can override previous entries (typically, entries obtained from
    included files).  Without this, conflicting definitions of the same
    timezone abbreviation are considered an error.
   </para>

   <para>
    In an unmodified installation, the file <filename>Default</filename> contains
    all the non-conflicting time zone abbreviations for most of the world.
    Additional files <filename>Australia</filename> and <filename>India</filename> are
    provided for those regions: these files first include the
    <literal>Default</literal> file and then add or modify abbreviations as needed.
   </para>

   <para>
    For reference purposes, a standard installation also contains files
    <filename>Africa.txt</filename>, <filename>America.txt</filename>, etc., containing
    information about every time zone abbreviation known to be in use
    according to the IANA timezone database.  The zone name
    definitions found in these files can be copied and pasted into a custom
    configuration file as needed.  Note that these files cannot be directly
    referenced as <varname>timezone_abbreviations</varname> settings, because of
    the dot embedded in their names.
   </para>

   <note>
    <para>
     If an error occurs while reading the time zone abbreviation set, no new
     value is applied and the old set is kept. If the error occurs while
     starting the database, startup fails.
    </para>
   </note>

   <caution>
    <para>
     Time zone abbreviations defined in the configuration file override
     non-timezone meanings built into <productname>PostgreSQL</productname>.
     For example, the <filename>Australia</filename> configuration file defines