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. Show More