Doomsday Weekday Trainer
John Conway’s Doomsday algorithm lets you compute the weekday of any date mentally, and with practice it takes seconds. The trick: in every year, an easy-to-remember set of dates — 4/4, 6/6, 8/8, 10/10, 12/12, pi day 3/14, the last day of February and friends — all fall on the same weekday, the year’s "doomsday". Find that one day (century anchor, plus the two-digit year, plus its quarter), then count from the nearest doomsday date to your target. This trainer deals random dates from 1800 to 2099: in practice mode every answer is followed by the full worked steps — anchor, year doomsday, nearest doomsday date, final count — and in quiz mode you race through ten dates while accuracy and average time are tracked. Storm mode is a survival run: 60 seconds on the clock, combo bonuses add time, misses cost it, and the dates climb a ten-level difficulty ladder from this century’s doomsdays to the leap-year January trap. Your best results are kept on this device.
Runs 100% in your browser — your puzzles never leave your device.
Read the full guide to this tool
Notes
- The doomsday dates to memorize: 4/4, 6/6, 8/8, 10/10, 12/12, then 5/9, 9/5, 7/11 and 11/7 ("9-to-5 at the 7-11"), 3/14, and the last day of February; January’s is the 3rd (4th in leap years).
- Century anchors repeat every 400 years: Tuesday for the 2000s, Wednesday for the 1900s, Friday for the 1800s. Year doomsday = anchor + (y + ⌊y/4⌋) mod 7, with y the two-digit year.
- Speed comes from casting out sevens early: 1987 → 87 + 21 = 108 is easier as 87→3, 21→0, 3+3(anchor) = Wednesday-ish arithmetic on numbers below 7.
- Storm mode mirrors the classic puzzle-storm pacing: combo bonuses of +3s at combo 5, +5s at 12, +7s at 20 and +10s at 30 and every 10 after, a 10-second malus per miss, and a difficulty ladder that rises one level (of ten) every 3 correct answers — from doomsday dates of easy 20xx years to Januaries and Februaries of arbitrary centuries, where the leap rule bites.
- Runs 100% in your browser — your words and puzzles never leave your device.
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