One Date in 12 Calendars

Pick a Gregorian date and the table writes that same day across fourteen rows: Gregorian, ISO 8601, Hebrew, Islamic in two variants, Persian, Indian national, Coptic, Ethiopian, Thai Buddhist, Minguo, Japanese, Chinese and Korean dangi. Fourteen rows for twelve calendars, because the Gregorian shows up twice, once in the usual form and once in ISO form, and the Islamic shows up as two different tables. Every row carries the spelled out date, the year, the month name, the day and the era abbreviation when the calendar has one: AH for the Hijra, AP for the solar Hijri, BE for the Buddhist era, Saka, Minguo, Reiwa. Above the table sits a summary of the Gregorian day with the weekday, the ISO week in the 2026-W31-6 shape, the position in the year, the Julian Day Number and whether the year is a leap year. The selector next to the date field swaps month and weekday names between Portuguese and English without touching a single number.

Each gap between the rows has a reason behind it. On 1 August 2026 the civil Islamic row reads 16 Safar 1448 while the Umm al-Qura row reads 18 Safar of the same year, two days apart on the very same Gregorian day, because the first follows a tabular thirty year rule and the second follows the official Saudi table. A lunar year runs about 354 days, so Ramadan walks back roughly eleven days per solar year. The Persian calendar does the opposite and stays locked to the Sun: the year opens at Nowruz, at the March equinox measured on the official Iranian meridian, so 21 March 2026 is 1 Farvardin 1405. The Hebrew calendar is lunisolar and closes the gap by inserting an extra month, Adar I, in seven years out of every nineteen. Coptic and Ethiopian both run twelve months of thirty days plus five or six loose days at the end, and Ethiopian year 2019 opens on 11 September 2026. Thai Buddhist and Minguo keep the Gregorian months and only shift the count, plus 543 and minus 1911. The Japanese calendar restarts with each emperor: Heisei 31 ended on 30 April 2019 and Reiwa 1 began the next morning. Chinese and dangi count in a sexagenary cycle, with 2026 landing on bing-wu, year 43 of 60, and slot in a leap month every few years, such as the second sixth month of 2025. Even the Gregorian input is itself a correction: the 1582 reform dropped ten days and stopped granting a leap day to century years that are not divisible by 400.

Nothing here is hand rolled calendar arithmetic. The answers come from the ICU built into your browser, reached through Intl.DateTimeFormat with the calendar extension, the same component your operating system uses to spell dates in any language. The date is built and read in UTC, otherwise your machine time zone would push the result one day back in Kiritimati and one day forward in Niue. After building each formatter the page checks which calendar the browser actually applied. If ICU ignores the request and quietly falls back to Gregorian, the row is reported as unavailable instead of passing a Gregorian date off as a Persian one. A browser missing the data for one calendar gets the same marker on that row while the rest keep working. What this page does not do: it never converts in the other direction, it has no old Julian calendar, no Maya and no French Republican one, it ignores time of day and time zones, and it does not settle the start of a month by moon sighting. The Islamic rows come from tables, and a local announcement can land a day earlier or later.