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Recipe 3.6 Finding the Difference of Two Dates
3.6.1 Problem
You want to find the elapsed time between two dates. For example, you want to tell a user
how long it's been since she last logged onto your site.
3.6.2 Solution
Convert both dates to epoch timestamps and subtract one from the other. Use this code to
separate the difference into weeks, days, hours, minutes, and seconds:
// 7:32:56 pm on May 10, 1965
$epoch_1 = mktime(19,32,56,5,10,1965);
// 4:29:11 am on November 20, 1962
$epoch_2 = mktime(4,29,11,11,20,1962);
$diff_seconds = $epoch_1 - $epoch_2;
$diff_weeks = floor($diff_seconds/604800);
$diff_seconds -= $diff_weeks * 604800;
$diff_days = floor($diff_seconds/86400);
$diff_seconds -= $diff_days * 86400;
$diff_hours = floor($diff_seconds/3600);
$diff_seconds -= $diff_hours * 3600;
$diff_minutes = floor($diff_seconds/60);
$diff_seconds -= $diff_minutes * 60;
print "The two dates have $diff_weeks weeks, $diff_days days, ";
print "$diff_hours hours, $diff_minutes minutes, and $diff_seconds ";
print "seconds elapsed between them.";
The two dates have 128 weeks, 6 days, 14 hours, 3 minutes,
and 45 seconds elapsed between them.
Note that the difference isn't divided into larger chunks than weeks (i.e., months or years)
because those chunks have variable length and wouldn't give an accurate count of the time
difference calculated.
3.6.3 Discussion
There are a few strange things going on here that you should be aware of. First of all, 1962
and 1965 precede the beginning of the epoch. Fortunately,
mktime( )
fails gracefully here
and produces negative epoch timestamps for each. This is okay because the absolute time
value of either of these questionable timestamps isn't necessary, just the difference between
the two. As long as epoch timestamps for the dates fall within the range of a signed integer,
their difference is calculated correctly.
Next, a wall clock (or calendar) reflects a slightly different amount of time change between
these two dates, because they are on different sides of a DST switch. The result subtracting
epoch timestamps gives is the correct amount of elapsed time, but the perceived human time
change is an hour off. For example, on the Sunday morning in April when DST is activated,
what's the difference between 1:30 A.M. and 4:30 A.M.? It seems like three hours, but the