Calendars and clocks ... they bring out the stupid. Check out this thread on a body-building forum for an outstanding example.
I once had a job at an electronic stock and option trading company, which was run by a very smart fellow with decades of experience. He and I once had a three hour debate about how time zones worked – he didn't believe it was true that at given moment there are either two or three dates, depending on where you are on the planet (the two dates occurs because of the international date line; the three dates possibility occurs for a more obscure reason – some very odd time zones in the Pacific Ocean islands). It didn't matter that I could give him articles, books, and even scientific papers discussing the phenomenon. Nothing I did could convince him, and because of that he wouldn't let me fix a bug in our software that affected traders operating from those aforementioned Pacific Ocean islands.
But at least he understood that there were seven days in a week, unlike one fellow on that thread!