Doomsday Trainer

Master the Doomsday Algorithm — calculate the day of the week for any date in history in seconds.

PLAY Build your streak and beat the clock. Every correct answer adds to your streak; one wrong answer ends it. Compete on the global leaderboard for Longest Streak and Fastest Average Speed.

BATTLE Challenge a real opponent to a live 1-on-1 race. Five dates, fastest and most accurate player wins. Elo-based ranking tracks your competitive record.

TRAIN Six methods to choose from: • Doomsday — the classic anchor-day approach • Odd + 11 — the Fong-Walters mental shortcut • The Wollin Shortcut — a memory-based system using vectorized doomsdays, the Square Knot Rule, and the Wollin Calamity Table (by u/Nillows) • Conway's Method — the original algorithm by John Conway • Mod 28 — reduce any year to under 28 before calculating • Custom Mode — pick your own date range, century, or decade

DRILL Targeted practice for each component: months, centuries, years, mod 7 arithmetic, and the full Wollin Shortcut drill suite.

TUTORIAL New to the algorithm? The built-in tutorial walks you through everything step by step.