Learn the doomsday algorithm

A small trainer for the Doomsday algorithm — a mental-math method for working out the weekday of any date. Four modes, each drilling one step of the method:

• Calculate — pick the weekday for a random date, then reveal the full derivation: the year math, the nearest memorized doomsday, the week-by-week walk to the target date, and the final answer.
• Anchor days — guess the anchor weekday for a random century.
• Leap years — guess whether a random year is a leap year.
• Doomsdays — a reference list of one memorable date per month that shares its weekday with the year's doomsday.

Development

npm install
npm run dev

Checks

npm run typecheck
npm run lint
npm run prettier
npm run test
npm run build

Deployment

Pushes to main are built and deployed to GitHub Pages automatically via .github/workflows/build-deploy.yaml. This requires Settings → Pages → Source to be set to GitHub Actions once, in the repository settings.