PHP DateTime Differences (Signed!)

18 Apr 2013 Joshua Chamberlain

Note to self: remember to add “%R” (or “%r”) when formatting a date difference in PHP. According to the PHP manual, the optional second parameter to DateTime::Diff() determines whether the difference between two dates is given as an absolute, i.e., forced to positive. That does NOT mean that “%a” returns a signed integer by default. Consider the following example:

$date1 = new DateTime('2013-04-10');
$date2 = new DateTime('2013-04-12');
$diff = $date1->diff($date2);
echo $diff->format('%a')."\n";

The “%a” format returns the total number of days, in this case 2. Since the second parameter is not set and thus defaults to false, you might think that this means $date2 is two days greater than $date1. Wrong – it could just as easily mean the opposite. Change $date2 to “2013-04-08” and you get the same result! Regardless of the optional $absolute parameter, “%a” (and its equivalent, $diff->days) never includes a sign. $diff->days and $diff->format('%a') will always return the absolute number of days between the two dates. If you care about which came first, use $diff->format('%r%a') instead.