The Doomsday algorithm is a way of calculating the day of the week of a given date. This algorithm for mental calculation was devised by John Conway after drawing inspiration from Lewis Carroll’s work on a perpetual calendar algorithm.
The algorithm is simple enough for anyone with basic arithmetic ability to do the calculations mentally. Conway can usually give the correct answer in under two seconds. To improve his speed, he practices his calendrical calculations on his computer, which is programmed to quiz him with random dates every time he logs on. src
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