I'm trying to work out how many seconds there are in a given time range.

So for instance if I have the following times: 16:36:00 - 16:37:38

I need to the value of 98 seconds to be placed within a 3rd cell. I've managed to get it down to 00:01:38 so far but I can't figure out how to get it broken down any further.

Any help will be greatly appreciated.