Recipe 3.11 Adding to or Subtracting from a Date
3.11.1 Problem
You need to
add or subtract an interval from a date.
3.11.2 Solution
Depending on how your date and interval are represented, use
strtotime( ) or some simple arithmetic.
If you have your date and interval in appropriate formats, the
easiest thing to do is use strtotime(
):
$birthday = 'March 10, 1975';
$whoopee_made = strtotime("$birthday - 9 months ago");
If your date in an epoch timestamp and you can express your interval
in seconds, subtract the interval from the timestamp:
$birthday = 163727100;
$gestation = 36 * 7 * 86400; // 36 weeks
$whoopee_made = $birthday - $gestation;
3.11.3 Discussion
Using strtotime( ) is good for intervals that are
of varying lengths, like months. If you can't use
strtotime( ), you can convert your date to an
epoch timestamp and add or subtract the appropriate interval in
seconds. This is mostly useful for intervals of a fixed time, such as
days or weeks:
$now = time( );
$next_week = $now + 7 * 86400;
Using this method, however, you can run into problems if the
endpoints of your interval are on different sides of a
DST switch. In this
case, one of your fixed length days isn't 86,400
seconds long; it's either 82,800 or 90,000 seconds
long, depending on the season. If you use UTC exclusively in your
application, you don't have to worry about this. But
if you have to use local time, you can count days without worrying
about this hiccup with Julian days. You can convert between epoch
timestamps and Julian days with unixtojd(
)
and jdtounix(
):
$now = time( );
$today = unixtojd($now);
$next_week = jdtounix($today + 7);
// don't forget to add back hours, minutes, and seconds
$next_week += 3600 * date('H',$now) + 60 * date('i',$now) + date('s',$now);
The functions unixtojd(
) and jdtounix(
) are part of PHP's calendar extension, so
they are only available if that extension is loaded.
3.11.4 See Also
Recipe 3.6 for finding the difference
between two dates in elapsed time; Recipe 3.7 for finding the difference between two dates
in Julian days; documentation on strtotime( ) at
http://www.php.net/strtotime, unixtojd(
) at http://www.php.net/unixtojd, and
jdtounix( ) at
http://www.php.net/jdtounix.
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