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</entry>

<entry><type>interval</type></entry>

<entry>Subtract arguments, producing a <quote>symbolic</> result that

- uses years and months</entry>

+ uses years and months</entry>, rather than just days

<entry><literal>age(timestamp '2001年04月10日', timestamp '1957年06月13日')</literal></entry>

<entry><literal>43 years 9 mons 27 days</literal></entry>

</row>

days</> because May has 31 days, while April has only 30.

</para>

+ <para>

+ Subtraction of dates and timestamps can also be complex. The most

+ accurate way to perform subtraction is to convert each value to a number

+ of seconds using <literal>EXTRACT(EPOCH FROM ...)</> and compute the

+ number of <emphasis>seconds</> between the two values. This will adjust

+ for the number of days in each month, timezone changes, and daylight

+ saving time adjustments. Operator subtraction of date or timestamp

+ values returns the number of days (24-hours) and hours/minutes/seconds

+ between the values, making the same adjustments. The <function>age</>

+ function returns years, months, days, and hours/minutes/seconds,

+ performing field-by-field subtraction and then adjusting for negative

+ field values. The following queries, produced with <literal>timezone

+ = 'US/Eastern'</> and including a daylight saving time change,

+ illustrates these issues:

+ </para>

+

+<screen>

+SELECT EXTRACT(EPOCH FROM timestamptz '2013年07月01日 12:00:00') -

+ EXTRACT(EPOCH FROM timestamptz '2013年03月01日 12:00:00');

+<lineannotation>Result: </lineannotation><computeroutput>10537200</computeroutput>

+SELECT (EXTRACT(EPOCH FROM timestamptz '2013年07月01日 12:00:00') -

+ EXTRACT(EPOCH FROM timestamptz '2013年03月01日 12:00:00'))

+ / 60 / 60 / 24;

+<lineannotation>Result: </lineannotation><computeroutput>121.958333333333</computeroutput>

+SELECT timestamptz '2013年07月01日 12:00:00' - timestamptz '2013年03月01日 12:00:00';

+<lineannotation>Result: </lineannotation><computeroutput>121 days 23:00:00</computeroutput>

+SELECT age(timestamptz '2013年07月01日 12:00:00', timestamptz '2013年03月01日 12:00:00');

+<lineannotation>Result: </lineannotation><computeroutput>4 mons</computeroutput>

+</screen>

+

<sect2 id="functions-datetime-extract">

<title><function>EXTRACT</function>, <function>date_part</function></title>