One hour has 60 minutes, and one minute has 60 seconds. To calculate the difference between two times, sort hours, minutes and seconds in appropriate pairs and begin deduction with groups of seconds. If you don’t have enough seconds in upper (bigger) time, borrow one minute (that is 60 seconds) from group of minutes. Continue in the same manner with pairs of minutes, and finally deduct hours.

Within the same day (or date), these calculations are simpler in 24-hour clock than in 12-hour clock, because you don’t have to think of A.M. and/or P.M.