All Calendars Now
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Sources & acknowledgements

All Calendars Now is a work of synthesis. Almost nothing here is original to it: every calendar is computed from methods worked out by scholars, astronomers, and the keepers of living traditions, and the invented ones belong to the writers and studios who dreamed them up. This page records that debt.

Calendar algorithms

The mathematics of the calendars — how to turn one date into another, when a leap month falls, where the moveable feasts land — follows the standard references, above all Nachum Dershowitz and Edward M. Reingold’s Calendrical Calculations, the definitive modern treatment. The astronomical moments — the equinoxes that open the Persian, Baháʼí and French Republican years, the phase of the moon, the hour of sunrise and sunset — are computed by the methods of Jean Meeus’s Astronomical Algorithms. The Hindu solar reckoning behind Vikram Samvat is derived from the sun’s sidereal longitude — the sankranti — by the same astronomy.

The living calendars

Many of the calendars still in daily use — the Hebrew, Islamic (Umm al-Qura), Persian, Indian national, Chinese, Ethiopian, Coptic, Buddhist, Japanese and Korean among them — come directly from your browser’s own internationalization engine (ICU), which draws on the Unicode Common Locale Data Repository (CLDR): the same data your operating system uses to render these dates. The site shows what your device already knows, so the results agree with your phone and your calendar app.

Correlations and reconstructions

Some calendars have no single official form and must be tied to ours by a chosen correlation. The Maya Long Count, Tzolkʼin and Haabʼ use the Goodman–Martínez–Thompson (GMT) correlation, the standard among Mayanists; the Aztec count follows the correlation of Alfonso Caso. The Balinese Pawukon is anchored to a verified recent date and run forward by its own ten interlocking cycles. Where scholars genuinely disagree, the site says so on the calendar’s own page rather than pretending to a certainty that isn’t there.

Indigenous and regional calendars

Several reckonings here belong to living cultures, and are shown with respect and — we hope — accuracy: the Balinese Pawukon; the Igbo four-day market week; the Akan day-names; the Hawaiian Kaulana Mahina, the reckoning of the moon; and the six seasons of the Noongar people of south-western Australia. These are other people’s traditions, not ours. We present them to be seen, and we welcome correction from those who keep them.

The imagined worlds

The fictional calendars are here out of affection, and they belong entirely to their creators. All Calendars Now is not affiliated with, authorised by, or endorsed by any of them; all names and trademarks remain the property of their owners, and the reckonings are computed, not copied — reproduced only to the small extent needed to tell the time in them. With gratitude to:

  • Dune — Frank Herbert and the Herbert estate
  • Foundation — Isaac Asimov and his estate
  • Star Wars — George Lucas and Lucasfilm
  • Star Trek — Gene Roddenberry and Paramount
  • Warhammer 40,000 — Games Workshop
  • The Lord of the Rings (the Shire and Gondor reckonings) — J.R.R. Tolkien and the Tolkien Estate
  • The Elder Scrolls — Bethesda Softworks
  • the Forgotten Realms — Ed Greenwood and Wizards of the Coast
  • The Witcher — Andrzej Sapkowski
  • The Wheel of Time — Robert Jordan and his estate
  • The Stormlight Archive (Roshar) — Brandon Sanderson
  • A Song of Ice and Fire (Westeros) — George R.R. Martin
  • Discworld — Terry Pratchett and the Pratchett estate
  • Stardew Valley — ConcernedApe (Eric Barone)

Typography and the web

The site is built with Next.js and React (both MIT-licensed, from Vercel and Meta) and hosted on Vercel, whose privacy-friendly analytics it uses. Its serif is Iowan Old Style and the system fonts already on your device; the share-card images are rendered with Lora, by Cyreal, under the SIL Open Font License. Our thanks to the maintainers of these open-source projects.

We aim to get this right, and to credit fairly. If you find an error in a calculation, a source we should add, or an attribution we’ve got wrong, please tell us — or write to hello@epiqara.com. See also the note on what is exact and what is estimated, and the glossary for the ideas the calculations rest on.