</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 '2001-04-10', timestamp '1957-06-13')</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 '2013-07-01 12:00:00') -

EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00');

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

SELECT (EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -

EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00'))

/ 60 / 60 / 24;

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

SELECT timestamptz '2013-07-01 12:00:00' - timestamptz '2013-03-01 12:00:00';

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

SELECT age(timestamptz '2013-07-01 12:00:00', timestamptz '2013-03-01 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>