These types of calculations usually use something called Unix epoch (the number of seconds since midnight UTC Jan 1st, 1970).

Check out POSIX, particulary mktime() to convert the time info in your format to Unix epoch. Then you can just take time differences in seconds. Use localtime should you want to convert back again. Since you're only interested in differences, that may not be important to you.

Many, many applications use epoch time internally as a timestamp. The reason: it simply (still) fits in a 4-byte integer and calculations on it are easy. This will all go well until somewhere early 2038. No doubt we will something of a Y2K scare again around then... ;-)

Hope this helps.

Liz