First time on the forum, need help fast!!

I have to write a programme to show the amount of elapsed time between two given times, for example, start time 14:59:59 - end time = 15:00:00. Elapsed time = 0:0:1. I know it is to do with the mod % function but the minutes are causing me problems. I'm new to this C business and would appreciate any help you kind people could give me!

Convert the times to seconds (hr*3600+min*60+sec) in order to calculate the difference.

Then use / and % to extract the difference in terms of hr, min and sec.

Your just to clever Salem, I spent ages messing about with time functions and go nowhere lol.