<para>

When adding an <type>interval</type> value to (or subtracting an

<type>interval</type> value from) a <type>timestamp with time zone</type>

value, the days component advances(or decrements) the date of the

value, the days component advances or decrements the date of the

<type>timestamp with time zone</type> by the indicated number of days.

Across daylight saving time changes (with the session time zone set to a

Across daylight saving time changes (when the session time zone is set to a

time zone that recognizes DST), this means <literal>interval '1 day'</literal>

does not necessarily equal <literal>interval '24 hours'</literal>.

For example, with the session time zone set to <literal>CST7CDT</literal>,

</para>

<para>

Note there can be ambiguity in the <literal>months</> returned by

<function>age</> because different months haveadifferentnumber of

Note there can be ambiguity in the <literal>months</>fieldreturned by

<function>age</> because different months have differentnumbers of

days. <productname>PostgreSQL</>'s approach uses the month from the

earlier of the two dates when calculating partial months. For example,

<literal>age('2004-06-01', '2004-04-30')</> uses April to yield

</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

Subtraction of dates and timestamps can also be complex. One conceptually

simple way to perform subtraction is to convert each value to a number

of seconds using <literal>EXTRACT(EPOCH FROM ...)</>, then subtract the

results; this produces 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

saving time adjustments. Subtraction of date or timestamp

values with the <quote><literal>-</></quote> operator

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:

field values. The following queries illustrate the differences in these

approaches. The sample results were produced with <literal>timezone

= 'US/Eastern'</>; there is a daylight saving time change between the

two dates used:

</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'))

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';