time calcs

Hello, I am trying to get a time calculation using:

        $tot_time = ($data['end_time'] - $data['start_time']);

The end_time & start_time are stored in a db as column type of time. I want to output the difference as a difference expressed in time format: 10:05:23 - 09:45:32 = 00:39:51 Do I need to break the stored time values apart and do calcs on each segment? I am not storing the calculated time. Larry R. Sieting