874
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.
Oh okay. Yeah I only have that rule of "every 4 years" in my head. I did some other programming exercise way back where we had some other rule, but I was thinking that it would end up being the same.