In junior high, we were tested on the proper way to calculate leap years. The popular assumption is that it’s every 4 years, but that’s not quite right. Actually, that’s just the starting point.

It’s a leap year if:

• The year is divisible by 4, and it’s not a century year
• The year is divisible by 4, and it’s a century year also divisible by 400.

In other words, 1900, while divisible by 4, is not divisible by 400, and so was not a leap year — nor will 2100 be one, either. However, 2000 was.

What’s sort of funny about this is that, to a first approximation, everyone in the sound of my voice could spend their whole life living on the simpler “every 4 years” rule and not miss a single one. From 1901 through 2099, it’s every 4 years, but only because we happen to be living through a once-every-400-years exception to the exception.