Just don't push Nick's analogy too far. It's a great illustration of what you need to do, except...
Adding variables that can roll over can cause problems. Subtracting, on the other hand is guaranteed to work. So, instead of thinking in terms of "I need to feed the cat in 8 hours, and it's 9:00 now, so I'll feed the cat at 17:00", think of it always in terms of "How long has it been since I fed the cat. If it's more than 8 hours, it's time to do it again."