An example would be if you want to find the month corresponding to a timestamp. The obvious thing to do is to scale the timestamp so that it gives you the number of months after the epoch, take that number % 12, and map 0 to January, 1 to February, etc. (assuming the epoch is in January). But if you try to do that with a timestamp from before the epoch, you'll end up with 0 for January, -11 for February, -10 for March, etc.