<listitem>

<para>

For <type>timestamp with time zone</type> values, the

number of seconds since 1970-01-01 00:00:00 UTC (can be negative);

number of seconds since 1970-01-01 00:00:00 UTC (negative for

timestamps before that);

for <type>date</type> and <type>timestamp</type> values, the

number of seconds since 1970-01-01 00:00:00 local time;

nominal number of seconds since 1970-01-01 00:00:00,

without regard to timezone or daylight-savings rules;

for <type>interval</type> values, the total number

of seconds in the interval

</para>

SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08');

<lineannotation>Result: </lineannotation><computeroutput>982384720.12</computeroutput>

SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40.12');

<lineannotation>Result: </lineannotation><computeroutput>982355920.12</computeroutput>

SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');

<lineannotation>Result: </lineannotation><computeroutput>442800</computeroutput>

</screen>

<para>

Here is how youcan convert an epoch value back to a time

stamp:

Youcan convert an epoch value back to a<type>timestamp withtime zone</type>

with <function>to_timestamp</function>:

</para>

<screen>

SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720.12 * INTERVAL '1 second';

SELECT to_timestamp(982384720.12);

<lineannotation>Result: </lineannotation><computeroutput>2001-02-17 04:38:40.12+00</computeroutput>

</screen>

<para>

(The <function>to_timestamp</> function encapsulates the above

conversion.)

Beware that applying <function>to_timestamp</function> to an epoch

extracted from a <type>date</type> or <type>timestamp</type> value

could produce a misleading result: the result will effectively

assume that the original value had been given in UTC, which might

not be the case.

</para>

</listitem>

</varlistentry>