3.10. Adding to or Subtracting from a Date
Problem
You need to add or subtract an interval from a date.
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;
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 ...
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