Decipher any date into its weekday — learn it, drill it, master it.
Daycipher is an offline-first, installable PWA that teaches you to compute the weekday of any date in your head, using Conway's Doomsday rule (devised by John Horton Conway). Learn the method step by step, drill it with instant step-by-step feedback, and track your progress over time — all on your phone, fully offline.
A good intro to the method: YouTube explanation.
Rebuilt from the original Python/Tkinter prototype (now in legacy/) into a
mobile-first React PWA. See the design spec and implementation plans in
docs/superpowers/.
Vite · React + TypeScript · IndexedDB (idb) · vite-plugin-pwa · Vitest. Deployed on Vercel.
Package manager: pnpm.
pnpm install
pnpm dev # http://localhost:5173
pnpm test # run the test suite
pnpm typecheck
pnpm lint
pnpm build # production build (generates the offline service worker)
pnpm preview # serve the production build locallysrc/engine/ Pure Doomsday-rule engine (weekday math), exhaustively tested
src/db/ On-device IndexedDB data layer + JSON backup
src/store/ Settings (zustand)
src/pwa/ Service-worker registration + update flow
src/features/ Today · Learn · Practice · Progress · Settings screens
src/components/ Shared UI (nav, install/update prompts)
src/styles/ Almanac design tokens + base styles
legacy/ Original Python/Tkinter prototype