<filename>postgresql.conf</filename>, or in any of the
     other standard ways described in <xref linkend="runtime-config"/>.
     There are also some special ways to set it:

     <itemizedlist>
      <listitem>
       <para>
        The <acronym>SQL</acronym> command <command>SET TIME ZONE</command>
        sets the time zone for the session.  This is an alternative spelling
        of <command>SET TIMEZONE TO</command> with a more SQL-spec-compatible syntax.
       </para>
      </listitem>

      <listitem>
       <para>
        The <envar>PGTZ</envar> environment variable is used by
        <application>libpq</application> clients
        to send a <command>SET TIME ZONE</command>
        command to the server upon connection.
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </sect2>

   <sect2 id="datatype-interval-input">
    <title>Interval Input</title>

    <indexterm>
     <primary>interval</primary>
    </indexterm>

     <para>
      <type>interval</type> values can be written using the following
      verbose syntax:

<synopsis>
<optional>@</optional> <replaceable>quantity</replaceable> <replaceable>unit</replaceable> <optional><replaceable>quantity</replaceable> <replaceable>unit</replaceable>...</optional> <optional><replaceable>direction</replaceable></optional>
</synopsis>

     where <replaceable>quantity</replaceable> is a number (possibly signed);
     <replaceable>unit</replaceable> is <literal>microsecond</literal>,
     <literal>millisecond</literal>, <literal>second</literal>,
     <literal>minute</literal>, <literal>hour</literal>, <literal>day</literal>,
     <literal>week</literal>, <literal>month</literal>, <literal>year</literal>,
     <literal>decade</literal>, <literal>century</literal>, <literal>millennium</literal>,
     or abbreviations or plurals of these units;
     <replaceable>direction</replaceable> can be <literal>ago</literal> or
     empty.  The at sign (<literal>@</literal>) is optional noise.  The amounts
     of the different units are implicitly added with appropriate
     sign accounting.  <literal>ago</literal> negates all the fields.
     This syntax is also used for interval output, if
     <xref linkend="guc-intervalstyle"/> is set to
     <literal>postgres_verbose</literal>.
    </para>

    <para>
     Quantities of days, hours, minutes, and seconds can be specified without
     explicit unit markings.  For example, <literal>'1 12:59:10'</literal> is read
     the same as <literal>'1 day 12 hours 59 min 10 sec'</literal>.  Also,
     a combination of years and months can be specified with a dash;
     for example <literal>'200-10'</literal> is read the same as <literal>'200 years
     10 months'</literal>.  (These shorter forms are in fact the only ones allowed
     by the <acronym>SQL</acronym> standard, and are used for output when
     <varname>IntervalStyle</varname> is set to <literal>sql_standard</literal>.)
    </para>

    <para>
     Interval values can also be written as ISO 8601 time intervals, using
     either the <quote>format with designators</quote> of the standard's section
     4.4.3.2 or the <quote>alternative format</quote> of section 4.4.3.3.  The
     format with designators looks like this:
<synopsis>
P <replaceable>quantity</replaceable> <replaceable>unit</replaceable> <optional> <replaceable>quantity</replaceable> <replaceable>unit</replaceable> ...</optional> <optional> T <optional> <replaceable>quantity</replaceable> <replaceable>unit</replaceable> ...</optional></optional>
</synopsis>
      The string must start with a <literal>P</literal>, and may include a
      <literal>T</literal> that introduces the time-of-day units.  The
      available unit abbreviations are given in <xref
      linkend="datatype-interval-iso8601-units"/>.  Units may be
      omitted, and may be specified in any order, but units smaller than
      a day must appear after <literal>T</literal>.  In particular, the meaning of
      <literal>M</literal> depends