How I learned of Dr. John Horton Conway's Doomsday Algorithm
Added 1997-08-29, Updated 2013-02-28
The Doomsday algorithm was created by John Horton Conway, an eminent mathematician, perhaps best known as the inventor of the Game of Life (see Link below).
I first heard about the Doomsday algorithm on November 27, 1982, on a CBC Radio program called Quirks and Quarks. Dr. Conway was interviewed by Jay Ingram, who's now Co-host and Producer of @discovery.ca. In those days Quirks and Quarks occasionally made typed transcripts available, and I sent away for one.
Dr. Conway had just published a book that year (co-authored by Elwyn R. Berlekamp and Richard K. Guy) called Winning Ways For Your Mathematical Plays, Volume 2: Games in Particular, Academic Press, London, 1982, ISBN 01-12-091102-7. The Doomsday algorithm is on pages 795-797, and the rest of the book is mainly about games, with substantial emphasis on their mathematical underpinnings. It is now available only in paperback.
In the original version of the Doomsday algorithm, the odd months were a bit harder to remember than "I work from 9-5 at the 7-11." You had to remember if the odd month was a long month or a short month. The 3rd, 5th, and 7th months are "long" because March, May, and July have 31 days, while the 9th and 11th months are "short" because September and November have only 30 days. You could remember "30 days hath September... and November" (but be careful because this old rhyme includes April and June which are even months). Anyway, for long odd months, Doomsday is the (N+4)th, while for short odd months, Doomsday is the (N-4)th. The mnemonic was long=add, short=subtract. Thus:
- March (3rd month, long) 3+4=7th is Doomsday
- May (5th month, long) 5+4=9th is Doomsday
- July (7th month, long) 7+4=11th is Doomsday
- September (9th month, short) 9-4=5th is Doomsday
- November (11th month, short) 11-4=7th is Doomsday
I'd agree that it's easier to remember "I work from 9-5 at the 7-11" together with "March 0th=7th".