Now, remember I said our year was just shy of 365.25 days? It's actually 365.24, but we use 365 and a quarter because that's close enough to work over the course of four years. Over the course of a hundred years, though, that difference adds up. If we simplify the math, we find that after 100 years, because of that .01 days we've been ignoring, we come out nearly a day ahead. So every 100 years we skip the leap day to catch back up again.