Calculating weekdays and dates with Go

Programming Snapshot – Go

Article from Issue 227/2019
Author(s):

Math wizards amaze audiences by naming the day of the week for any date called out by the audience. Mike Schilli writes a training program in Go to turn amateurs into headline performers.

Math geniuses can do this: Someone from the audience calls out "December 12, 2019," and the numbers wizard announces "Thursday!" after just a few seconds. How did he do that? Does the entertainer have a photographic memory, or some kind of calendar function built into his head? The solution is surprisingly simple: He only has to go through a few rules that are easy to remember and, with a little bit of practice, can come up with the day of the week for any given date.

Years ago in this column, I introduced a similar mental arithmetic method for calculating weekdays, but with more elaborate steps [1]. A reader then replied that the method was unnecessarily complex and referred me to the simpler Doomsday rule [2], which I will use here to create a Go training program for weekday prediction.

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