So for example, if the HH from the file is 00 and the current HH from unix is 20 the diff is 20, but in reality the time diff is only 4.

Says who? I say the difference between 00:00 and 20:00 is 20 hours, not 4 hours. Or maybe I change my mind and say the difference is 748 hours. I'm still right, and you're still wrong.

The point is you didn't provide any information about what days the two different hours are in or how you're determining which days they're in. Or you neglected to state a critical assumption about your problem.

Jim