Not exactly a joke, but a true story dealing with time travel.
Many years ago I was trying to book a flight on United and I made a mistake and transposed the departure and return dates. The site popped up a window that said "we have not yet perfected time travel. Please check your dates and try again." I kid you not. Never had that happen again and, silly me, I didn't think to take a screen shot.
I now return you to your time travel joke thread, already in progress.