To find out time elapsed i usually use time() instead of date() and formatted time stamps.
Then get the difference between the latter value and the earlier value and format accordingly. time() is differently not a replacement for date() but it totally helps when calculating elapsed time.
example:
The value of time() looks something like this 1274467343 increments every second. So you could have $erlierTime with value 1274467343 and $latterTime with value 1274467500, then just do $latterTime - $erlierTime to get time elapsed in seconds.
