Calculations Involving Durations in hh:mm, NOT Times

Hello,

I am in charge of statistics at a virtual air traffic control organization (VATSIM.net if you want to learn more). We have a network on which members' session durations are tracked in hh:mm. I was wondering if there was a way that I could input those session durations into a cell next to their name, input their totals for last month in the field next to it, and then use an IF statement as such: IF([cell containing this month's online time]-[cell containing last month's online time]=>0,"Up","Down"). The idea is that if this month's online time exceeds last month's online time, a third cell echoes (using PHP terms here...hehe) "up," and if not, "down," meaning their online time increased this month.

The part on which I am confused is how to format and use durations, not time, in hh:mm. Excel seems to want to use, for instance, 12:00 (which would be twelve hours and zero minutes) as 12:00 P.M.. I just want it to indicate twelve hours, not twelve o'clock. Similarly, 67:00 would be sixty-seven hours, not an imaginary 67 o'clock.

Many thanks!