You can use the the strtotime function to turn the time to integers and subtract them.
$time1 = strtotime("2008-12-13 10:42:00");
$time2 = strtotime("2010-10-20 08:10:00");
$diff = $time2-$time1;
// the difference in int. then you can divide by 60,60,24 and
// so on to get the h:m:s out of it