Doomsday Drill

The story

I picked up John Conway's Doomsday rule from an It's Okay to be Smart video and now I can tell you that July 4, 1776 was a Thursday. The math is short once the anchors are memorized, but it only gets quick with reps, so I built the reps. The page deals you a random date, you work it out in your head, and you hit reveal when you think you have it. It keeps a running tally, a streak, and a timer so you can see whether you are actually getting faster, and the month and century anchors sit in a cheatsheet right under the card for when they have not stuck yet.

What I learned

1. 01The whole trick is three numbers: the century anchor, the year offset, and the doomsday date for the month.
2. 02The page reveals first and lets you say whether you got it, since grading a number you worked out in your head has to run on the honor system to mean anything.
3. 03Weekday math is a timezone trap, so every date is built and read in UTC and never slides a day depending on where the page is open.
4. 04A drill lives or dies on how fast you can go again, so the whole loop is one key press: space to reveal, y or n to record it.
5. 05The tally lives in localStorage, which is the right amount of backend for a practice tool nobody needs an account for.