I ran into the same problem as you. I was trying to come up with a way
to print out my schedule and taking it a step further, wanted to
estimate how much my net pay would be. The formula I came up with with
the result in cell A3 is:

=IF((A2-A1)*24<0,((A2-A1)*24)+24,(A2-A1)*24)

As in the other suggestions, you need to use military 24 hour time
format to input the time.

Hope it works for you.
TCrow