PHP : Dates and Times - [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.