868
xkcd #2897: Light Leap Years
(imgs.xkcd.com)
Alt text:
When Pope Gregory XIII briefly shortened the light-year in 1582, it led to navigational chaos and the loss of several Papal starships.
No, years divisible by 100 aren't leap years, except if they're also divisible by 400.
what is this, some sort of FizzBuzz calendar?
Oh right, I had some programming exercise about this, way back.