HI, HELP!

I'm trying to generate the formula to subtract 24hr time clocks. If I want
to work out the hours between 0500 and 1600 then it's a simple case of
subtraction - however, the problem is when I try and reverse this. I'm trying
to calculate a formula to work out the hours between, for example, 1600 and
0630 - the formula I've tried using doesn't work so is obviously wrong.

Please help!