Any date. Any year.
In your head.
Doomsday teaches the algorithm that turns a date into a weekday in a few seconds of mental arithmetic. No calendar, no phone, no trick — four small steps, learnable in a couple of weeks.
25 December 2031 — which weekday?
1. The 2000s anchor is Tuesday.
2. Year 31 shifts it three days on: Friday.
3. So the 12th of December is a Friday.
4. The 25th is thirteen days later, which is six past a whole week.
Thursday.
Three ways in
Learn
A course of 48 lessons across 11 chapters, each ending on a question that makes you use what it just taught, plus seven Extras on why any of it works.
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 feeding one rating and the leaderboards.
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