Leap Days

We have them every 4 years, but not every 100 years, except when it’s every 400 years. [The Bad Astronomer has the explanation.][1] Here’s what it comes down to: our day is not really 24 hours long.

[1]: http://blogs.discovermagazine.com/badastronomy/2012/02/29/why-we-have-leap-days-2/