International Date Line crossing -- the Magellan problem
Weird request, but consider the following. I cross the dateline going west on a round the world trip. I lose a day, from a calendar standpoint (forever, if I don't return going east). I had two Christmases once when I traveled east around the world.
Not a common problem, I know.... But I'd love to be able to hack the month I'm traveling in so that it has two or zero of a particular day when I travel across the date line.