# Doomsday: Links

#### Links to resources on the Web for additional information about the Doomsday Algorithm

##### Added 1997-08-29, Updated 2017-01-03

The following web sites are about or include descriptions of Dr. Conway's Doomsday algorithm.

Doomsday Rule — the Wikipedia entry; very comprehensive.

What Day Is Doomsday? How to Mentally Calculate the Day of the Week for Any Date, October 2011 article in Scientific American.

First Sunday Doomsday Algorithm explains a modification to the Doomsday algorithm using the "first Sunday of the month" and the "Odd+11" rule.

If you're into heavy math, see Methods for Accelerating Conway's Doomsday Algorithm (part 1) PDF

Simon Cassidy comments on the "Hand" in the context of the Dee-Cecil calendar.

C.07.2 Can I calculate the date of Easter? explains Conway's algorithm for Easter, and gives another explanation of his Doomsday algorithm; includes the remark "Note to non-US readers: 'Seven-Eleven' is the name of a ubiquitous chain of convenience stores." Reader

**Richard Ezell**wrote to me in 2004 to report that this explanation may not really be necessary, as he had seen four 7-11 stores in a seven block stretch in Bangkok, Thailand.AST 309-TIME; What is the day of the week, given any date? contains notes by William H. Jefferys for a school course on time, with another explanation of the Doomsday algorithm (examples are from 1997).

The Doomsday Rule for Fortnights, by Jim Blowers, gives calculations for Doomsday based on 14-day periods.

Kate Larson's Mathematical poem to calculate the "day of the week" for any day of any year is a beautiful, whimsical poem, attributed to Dr. Conway, which describes the algorithm completely, including both Gregorian and Julian century adjustments. (Note: link goes to archive.org, as the original has dropped off the Web.)

The Doomsday Algorithm, by Mike Husband, is a Powerpoint presentation. (Note: link goes to archive.org, as the original has dropped off the Web.)

Any Day of the Week Using the Doomsday Rule, by Paul J. Weiss, is a C++ implementation with downloadable code.

For more information about Dr. Conway, see:

John Horton Conway: the world's most charismatic mathematician. "John Horton Conway is a cross between Archimedes, Mick Jagger and Salvador Dalí. For many years, he worried that his obsession with playing silly games was ruining his career – until he realised that it could lead to extraordinary discoveries." Story by Siobhan Roberts, author of Genius at Play, The Curious Mind of John Horton Conway published by Bloomsbury, 2015.

Inside the mind of 'mathemagician' John Horton Conway. In this Toronto Star excerpt from her biography, Genius at Play, author Siobhan Roberts introduces readers to a distinguished scholar who claims never to have worked a day in his life.

Interview with John Horton Conway (PDF). Edited version of an interview with John Horton Conway conducted in July 2011 at the first International Mathematical Summer School for Students at Jacobs University, Bremen, Germany

Not Just Fun and Games April 1999 Scientific American profile of John H. Conway. (

**Note:**this article is now available online only if you purchase the digital edition.)Charles Seife's Mathemagician -- an amusing article about John Horton Conway.

John Conway's Game of Life by Stephen Stuart -- an interactive version that you can play via your web browser.

### Interesting calendar links

Download a 12 sided calendar for any year. Print the calendar on a sheet of paper, and then cut it out and fold it into a dodecahedron!

A History of Time and Ancient Calendars by Niclas Marie

For links to other calendar sites, see my Calendar Links page; CAUTION, this page of links has not been updated since 2003!

### General mathematics links

## Knot a Braid of Links

The Doomsday Algorithm was "latest link in the braid" for the week of April 6-12, 1999.

"This page will teach you a simple algorithm to calculate mentally the day of the week corresponding to any given date. Give it a try, it's quite rewarding! The page features clear instructions, examples, and mnemonic tricks."

**KaBoL** is a "cool math site of the week" service to the
mathematics community provided by the
Canadian Mathematical Society.