Doomsday Game
A playful little trainer for learning Conway’s trick for finding the day of the week for any date.
A tiny calendar brain game
What day of the week was August 8, 2088? This little trainer teaches John Conway’s Doomsday Rule, a mental method for answering questions like that without checking a calendar.
There are two ways in: Learn walks through the method, and Practice throws out random dates for quick timed rounds. Get one right and keep the streak going; miss one and the game shows how the answer was worked out.
How the date trick works here
The calculation is broken into friendly chunks: find the century’s anchor day, work out the year’s Doomsday, use the month’s anchor date, then count to the date you were given. In practice mode, the game reveals each part after an answer, so a wrong guess still teaches you something.
Why I made it
I made this as a playful build for one of my Instagram videos. It is a small, self-contained experiment: learn a surprisingly useful mental trick, then see how quickly you can do it. No grand calendar platform here—just dates, a timer, and the satisfaction of getting a weekday right in your head.
Under the hood
The app is built with Next.js, React, and TypeScript. The date calculation lives in a small standalone module, which returns both the weekday and the intermediate values used to reach it. That breakdown powers the explanations in the learning and practice flows.