'1'</literal> means 1 second.  Also, field values
     <quote>to the right</quote> of the least significant field allowed by the
     <replaceable>fields</replaceable> specification are silently discarded.  For
     example, writing <literal>INTERVAL '1 day 2:03:04' HOUR TO MINUTE</literal>
     results in dropping the seconds field, but not the day field.
    </para>

    <para>
     According to the <acronym>SQL</acronym> standard all fields of an interval
     value must have the same sign, so a leading negative sign applies to all
     fields; for example the negative sign in the interval literal
     <literal>'-1 2:03:04'</literal> applies to both the days and hour/minute/second
     parts.  <productname>PostgreSQL</productname> allows the fields to have different
     signs, and traditionally treats each field in the textual representation
     as independently signed, so that the hour/minute/second part is
     considered positive in this example.  If <varname>IntervalStyle</varname> is
     set to <literal>sql_standard</literal> then a leading sign is considered
     to apply to all fields (but only if no additional signs appear).
     Otherwise the traditional <productname>PostgreSQL</productname> interpretation is
     used.  To avoid ambiguity, it's recommended to attach an explicit sign
     to each field if any field is negative.
    </para>

    <para>
     Internally, <type>interval</type> values are stored as three integral
     fields: months, days, and microseconds.  These fields are kept
     separate because the number of days in a month varies, while a day
     can have 23 or 25 hours if a daylight savings time transition is
     involved.  An interval input string that uses other units is
     normalized into this format, and then reconstructed in a standardized
     way for output, for example:

<programlisting>
SELECT '2 years 15 months 100 weeks 99 hours 123456789 milliseconds'::interval;
               interval
---------------------------------------
 3 years 3 mons 700 days 133:17:36.789
</programlisting>

     Here weeks, which are understood as <quote>7 days</quote>, have been
     kept separate, while the smaller and larger time units were
     combined and normalized.
    </para>

    <para>
     Input field values can have fractional parts, for example <literal>'1.5
     weeks'</literal> or <literal>'01:02:03.45'</literal>.  However,
     because <type>interval</type> internally stores only integral fields,
     fractional values must be converted into smaller
     units.  Fractional parts of units greater than months are rounded to
     be an integer number of months, e.g. <literal>'1.5 years'</literal>
     becomes <literal>'1 year 6 mons'</literal>.  Fractional parts of
     weeks and days are computed to be an integer number of days and
     microseconds, assuming 30 days per month and 24 hours per day, e.g.,
     <literal>'1.75 months'</literal> becomes <literal>1 mon 22 days
     12:00:00</literal>.  Only seconds will ever be shown as fractional
     on output.
    </para>

    <para>
     <xref linkend="datatype-interval-input-examples"/> shows some examples
     of valid <type>interval</type> input.
    </para>

     <table id="datatype-interval-input-examples">
      <title>Interval Input</title>
      <tgroup cols="2">
       <thead>
        <row>
         <entry>Example</entry>
         <entry>Description</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry><literal>1-2</literal></entry>
         <entry>SQL standard format: 1 year 2 months</entry>
        </row>
        <row>
         <entry><literal>3 4:05:06</literal></entry>
         <entry>SQL standard format: 3 days 4 hours 5 minutes 6 seconds</entry>
        </row>
        <row>
         <entry><literal>1 year 2 months 3 days 4 hours 5 minutes 6 seconds</literal></entry>
         <entry>Traditional Postgres format: 1 year 2 months 3 days 4 hours 5 minutes 6 seconds</entry>
        </row>
        <row>