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All Calendars Now
Guides
The 61 readings on the home page tell you what today is. These explain the machinery underneath — why a lunar year drifts, what a leap day is correcting, and why no two calendars agree on which year it is.
Why the Islamic calendar drifts
Ramadan moves about eleven days earlier each year while the Hebrew and Chinese new years stay put. The reason is one design decision, made deliberately.
How leap years work
Every four years, except century years, except those divisible by 400. Where that rule comes from, what it is correcting, and how other calendars solve the same problem.
Why calendars start where they do
A calendar has to count from something. What each one chose — a founding, a flight, a birth, a coronation, a computer being switched on — and why the choice outlasts the reason.
What a calendar is for
Unix time and the Balinese Pawukon are both on this site and they have almost nothing in common. One counts, the other cycles — and the difference explains what a calendar is actually built to do.
Looking for a definition rather than an explanation? The glossary covers the vocabulary, and the FAQ answers the shorter questions.