Doomsday MethodThe Doomsday method: Name the weekday of any date. In your head.
Someone names a date, you name its weekday in seconds, with no calendar. John Conway's method makes it four small steps, and the Doomsday app for iPhone teaches you all of them.
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The trick that makes it possible
Every year, 4/4, 6/6, 8/8, 10/10 and 12/12 share one weekday: the year's doomsday.
Learn one weekday, and you have the whole year.
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Your turn.
Any date from 1583 to 2599. Not sure how yet? Answer anyway and look at the working, or read how it works first.
Your date
Which weekday is this?
Want these right every time? The Doomsday app's free lessons teach each step →
How it works
The Doomsday algorithm in four steps
Steps 1 and 2 give you the year's doomsday. Steps 3 and 4 turn it into an answer. For a date this year, start at step 3.
Century anchor
One weekday per century. Four of them, and they repeat every 400 years.
Year offset
A number from 0 to 6 from the last two digits of the year. Add it to the anchor.
Month doomsday
One easy date in every month that always lands on the year's doomsday.
Count
Step to your date in sevens, which are free, then a few days either way.
Doomsday for iPhone
Go further in the app
From your first weekday to Julian dates and Easter, with a rating that tracks how good you actually are.
Learn
A course of 48 lessons across 11 chapters, each ending on a question that makes you use what it just taught, plus Extras on why any of it works. The first 16 lessons are free.
Train
Ten drills: three fundamentals, one for each step of the method, then reverse dates, the Julian calendar and Easter. Stars fill as your clean answers do.
Play
Classic with no clock, a Daily Challenge of five dates the whole world gets the same, and Time Attack. All of it feeds one rating and Game Center leaderboards.
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- One-time unlock, no subscription
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Questions
Frequently asked
Who invented the Doomsday method?
The mathematician John Horton Conway devised it in 1973, building on a method Lewis Carroll published in 1887. Conway named each year's shared weekday its doomsday. You can watch Conway explain it himself.
How long does it take to learn?
Dates in the current year take an afternoon, because you only need the twelve month dates and a short count. Any date in the 1900s and 2000s takes a couple of weeks of a few minutes a day. Speed comes with practice after that.
Do I need a good memory?
No. The whole method is about a dozen facts: a weekday for each century and one easy date per month, most of which pair up. The arithmetic never goes beyond adding small numbers and taking away sevens.
Does it work for any year?
It works for any date in the Gregorian calendar, past or future, because the century anchors repeat every 400 years. For dates before a country switched from the Julian calendar, such as Britain before September 1752, you need a small Julian adjustment, which the app teaches.
Is the Doomsday app free?
It is free to start: the first 16 lessons, three drills, 30 Classic dates a day and the Daily Challenge every day. Doomsday Pro is a one-time purchase that unlocks the full course, every drill and Time Attack. There is no subscription.