Everyone knows that there are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and 365 days in a year. But every now and then, something weird happens, and there are 366 days in a year. The reason for this is that a year is in fact about 365.25 days, but it would complicate people’s New Year’s Eve celebrations if the New Year didn’t start at precisely midnight every year. So we add an extra day to the end of February every four years, despite the fact that it would be much cooler if that extra day could be added to Christmas break instead. Obviously, whoever made that decision (Pope Gregory XIII, I believe) didn’t really think that part through very well. The weird part is that a year isn’t exactly 365.25 days either. It’s more like 365.242, so adding an extra day every four years will eventually get us a little ahead. The obvious solution to this dilemma was to make a rule that, if a year is divisible by 100, it’s only a leap year if it’s also divisible by 400. (Hence, 2000 was a leap year, but 1900 wasn’t) It’s almost as weird as daylight’s savings. But not quite.

On the other hand, it might be better just to think of February 29 as being a totally random day that arbitrarily appears on our calendar once every four years for no particular reason. That way, we can pretend that we’re all time travelers.