Doomsday drill · calendar math
What day of the week was it?
You get a date, you work it out in your head, and you hit reveal once you think you have it.
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Cheatsheet ▾
Doomsday by month
1/3 (1/4)2/28 (2/29)3/144/45/96/67/118/89/510/1011/712/12
The dates in parentheses are the leap year versions of January and February.
Doomsday by century
- 1600 · 2000 · 2400
- Tuesday
- 1700 · 2100 · 2500
- Sunday
- 1800 · 2200 · 2600
- Friday
- 1900 · 2300
- Wednesday
The steps
- 01Start with the anchor day for the century, so 1900s is Wednesday and 2000s is Tuesday.
- 02Take the last two digits of the year and divide by 12, then keep the quotient, the remainder, and the remainder divided by 4 with the fraction thrown away.
- 03Add those three numbers to the anchor and subtract 7s until you are under 7. That weekday is the doomsday for the whole year.
- 04Every date in the month list above lands on that weekday, so count forward or back from the closest one to your date.
The trick is John Conway's Doomsday rule. I learned it from the It's Okay to be Smart video above, and the printable sheet is theirs.