All Calendars Now
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Sources & acknowledgments

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, as is the sun’s place in the zodiac, tropical and sidereal.

Calendars that are published, not computed

Some calendars cannot be worked out from first principles, because the people who keep them do not work them out that way, and for those the printed calendar is the authority and this site follows it. West Bengal’s traditional Bengali months open by the sun of the Surya Siddhanta, a first-millennium astronomical treatise, and not by the sun overhead — reckoned here after Dershowitz and Reingold’s treatment of it, and checked month by month against the almanacs published at prokerala.com and bengalicalendar.com. Bangladesh’s calendar follows the Bangla Academy’s revision, as amended in 2019. Nepal’s Bikram Sambat is settled each year by a government almanac committee and issued about a year ahead, so its month lengths here are the published ones — cross-checked between Hamro Patro, ashesh.com.np and the open-source nepali-date-converter — and any year Nepal has not yet printed is marked as a projection. The Coptic month names are the Bohairic ones the Coptic Church reads its liturgy in.

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.

The Thai lunisolar calendar

The reckoning that fixes Vesak and the uposatha days is computed here by the traditional Suriyayart arithmetic, following the scholarship of J.C. Eade — The Calendrical Systems of Mainland South-East Asia and his papers for the Siam Society — together with Lars Gislén and Eade’s survey of the calendars of Burma, Thailand, Laos and Cambodia in the Journal of Astronomical History and Heritage. The multi-year adjustments that settle a year’s shape before a date can be read from it are subtle, and Mark Hollow’s pythaidate is the clearest working statement of them; this implementation agrees with it on every date tested. It is deliberately arithmetic rather than astronomical — a version built on true new moons puts Asalha Bucha 2026 a day late, because the calendar’s months open where the arithmetic opens them and the observances follow the arithmetic.

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, authorized 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)

Three of those calendars also appear in the script of their own world — on the card, over the month grid and on the printed sheet. The two Middle-earth reckonings are written in Tengwar, J.R.R. Tolkien’s script, spelled by Tecendil, Arno Gourdol’s transcriber — Gondor’s Quenya month and weekday names in the classical Quenya mode, the Shire’s in Tolkien’s English mode, and the days of the month in tengwar numerals, counted in twelves with the smallest digit first, as the Eldar reckoned. They are set in Alcarin Tengwar by Toshi Omagari. The Star Wars date is set in Aurebesh, the alphabet of that galaxy, in the Aurebesh font by SilvinoR. Both fonts are used unmodified under the SIL Open Font License, which travels with them (Alcarin Tengwar, Aurebesh); the scripts themselves remain their creators’. If a font does not reach your device, the line is simply left out.

The world map

The coastlines on the daylight map are Natural Earth, the public-domain cartographic dataset built and maintained by volunteers under the auspices of the North American Cartographic Information Society. We use the 1:110m land layer, simplified for a map this size and baked into the page, so viewing it asks nothing of any other server. Natural Earth is released into the public domain; its authors ask for no permission and no attribution, which seems to us all the more reason to give it.

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 contact [at] allcalendarsnow.com. See also the note on what is exact and what is estimated, and the glossary for the ideas the calculations rest on.