Another possibility is to compare the times. If the finish time is numerically LESS than the start time add 24 hrs to it & then subtract the start time.
That should compute the correct period.
Example of the method:
Start at 2200, finish at 0200
0200 is < 2200, therefore add 24 hrs = 26 hrs.
Subtract 22 hrs from 26 hrs =4 hrs period
Afraid I'm too lazy tonight to turn the above into an Excel formula though!