Doomsday Rule (Conway)

John Conway devised this method in 1973 to work out the weekday of any date without paper. He could do it in under two seconds himself, and kept a program on his computer that quizzed him with random dates before letting him read his email. The central idea is that in any given year, a handful of dates scattered across the twelve months always fall on the same weekday — that year's Doomsday.

The anchor dates are easy to hold because most of them are repetitions: 4/4, 6/6, 8/8, 10/10 and 12/12. The rest come in reversed pairs — 5/9 and 9/5, 7/11 and 11/7, which Americans remember as working nine to five at a 7-Eleven. That leaves January, the 3rd in a common year and the 4th in a leap year, February, the last day of the month, and March, the 14th, which is pi day.

The page does the arithmetic in three steps and shows each one: the century anchor, the year's Doomsday within it, and the distance in days to the date you asked about. Since the result is checked against the browser clock, the tool doubles as practice — work it out in your head, type the date, and see whether you got it. The four possible centuries repeat every 400 years, which is the Gregorian cycle.