It amused me to give the French Revolutionary calendar to the ancien regime. The indictions counted in groups of seven is actually a feature of Byzantine time-keeping; I just extrapolated--what would be the cultural effects of a religion that encouraged thinking in base-7--as a thought experiment.

The even simpler reason is that when I started writing these books in college, I was writing them just for me, so there was no reason not to throw in all the wild and crazy stuff I could find. And it never occurred to me that it would cause readers as much difficulty as apparently it does.