Well, if I had to guess, I'm assuming whoever came up with our calendar system probably had the same idea as your Pope Gregory XIII. Maybe not so much the years, but I can't help but notice that your years are based on the birth of... Jesus? That was his name, right? So I guess it makes sense that a pope had this idea in your universes.

It's just a guess though. I don't have any books from home I can pull up, so.

[ Has Gohan mentioned he's a huge nerd yet? ]