@Gargron The French Revolutionary Calendar had 10 days a week. They used it for 12 years. As far as I know, it did not lead to space-time difficulties, but it did lead to Napoleon, who was a bit of a disruption in himself. So if a French John Lennon in say about 1800 sang, "Oooh I need your love babe, ten days a week," it would not be hyperbole, but perhaps just a manifestation of the "many worlds" version of multiverse theory.