The idea
Every year has one weekday, its doomsday, that a set of easy dates always share. Know a year's doomsday and any date in it is a short count away.
Every year, one weekday
4 April, 6 June, 8 August, 10 October and 12 December always share it.
Learn one weekday, and you have the whole year.
1. Century anchor
Each century starts from a fixed weekday. There are four, and they repeat every 400 years.
2. Year offset
Take the last two digits of the year, add a quarter of them (dropping the remainder), and take away sevens. Add what's left, 0 to 6, to the anchor. That's the year's doomsday.
3. Month doomsday
One date in each month always lands on the doomsday:
The even months double up. For May, July, September and November: "I work 9-to-5 at the 7-Eleven" (5/9, 9/5, 7/11, 11/7).
4. Count
Step from that date to yours. Sevens are free, since a week later is the same weekday, so you're never more than three days away.
All together: 20 July 1969
The moon landing was on a Sunday. For a date this year, skip straight to step 3: 2026's doomsday is Saturday.
Learn it properly. The Doomsday app turns these four steps into 48 short lessons with drills. The first 16 are free.
Try any date
Where it comes from
Conway, the mathematician behind the Game of Life, published the method in 1973 as Tomorrow is the Day After Doomsday in the Cambridge journal Eureka. He built on a rule Lewis Carroll had sent to Nature in 1887. It works for any date in the Gregorian calendar, which began in 1582.
Watch and read more
- John H. Conway – DoomsdayVideo · Conway explains his own method, filmed by the Mathematical Association of America
- The Doomsday AlgorithmVideo · Numberphile, with James Grime
- What Day Is Doomsday?Article · Chamberlain Fong in Scientific American, 2011
- Methods for Accelerating Conway's Doomsday AlgorithmPaper · Fong and Walters, 2010: the Odd + 11 year offset
- Doomsday ruleWikipedia · the method, its variants and its history
Now make it fast
Speed comes from practice. Try dates in the free trainer and look at the working whenever you miss. The app adds faster year-offset methods such as Odd + 11 and a rating that tracks your speed.