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.
There hasn't been a Papal starship yet. I'm pretty sure he could Christen one, or delegate that authority to the bishop of the moon, an actual thing that technically exists.