Calculating time in a 24 hour format

There are two ways to do it - the first one you won't like because it requires entering the date as well as the time. That's the usual way Numbers expects times to be entered, and if you were to do that you could perform a simple subtraction as it appears you're trying to do.

The other way is to test for a negative result but you can't do that directly since it won't work for duration values. You can perform that test using the TIMEVALUE function, and correct for negative results by adding 24 hours. Restore a duration value with the DURATION function.

Here is the function and its results:

Block times work the same way, just use the out and in times instead of on / in:

Let me know if that works for you.