Formatting cells for elapsed time values, not time of day

I am doing some research and we take time values from a stopwatch (mm:ss as elapsed time), and we've got about 50 values or so per subject. I would like to convert those values to seconds (elapsed).

(Problem 1) When I put the values in excel as "15:20" to indicate 15 minutes and 20 seconds have passed on the stopwatch excel assumes I mean 3:20 PM. I've tried custom formatting the cell [mm]:ss, but excel still assumes the input I give it is hh:mm AM/PM and formats it accordingly. For example, the above input would give "920:00".

(Problem 2) When Problem 1 is solved I would like to convert those mm:ss elapsed time values into seconds.

Thanks for any help you can provide!