Date Decoder is a free tool from Decoders for exploring how a single day is named across the calendars of the world: religious, civil, astronomical and esoteric. It was built for students of history, timekeeping and the mysteries of the cycles.
All calculations run in your browser. The Julian Day Number, Julian, Ethiopian, Coptic, Mayan Long Count/Tzolk'in/Haab, 13 Moon Dreamspell, Anno Lucis, ISO week and day-of-year conversions use original code written for this site. The Ethiopian and Coptic arithmetic follows the published algorithms in Reingold & Dershowitz, Calendrical Calculations. The Mayan conversion uses the GMT correlation (584283). The 13 Moon count is anchored on July 26, 2013 = Kin 164 (Yellow Galactic Seed), skipping February 29 (0.0 Hunab Ku), with July 25 as the Day Out of Time.
The Chinese, Hebrew, Islamic (Umm al-Qura), Persian, Indian National, Buddhist, Japanese and Minguo dates come from your browser's built-in Intl.DateTimeFormat calendar support (the open-source ICU library, Unicode License). No third-party conversion code was copied.
Lunar calendars such as the Islamic and Hebrew can vary by a day depending on local moon sighting or tradition. Use this site for study and curiosity, not as a religious authority.
The site is built from small plug-in files, one per calendar, all fed from the Julian Day Number. That is also how the "From Julian" tab converts exactly, rather than adding a fixed 13 days (the gap was 10 days in 1582 and becomes 14 in 2100).
Fonts: Marcellus and Inter, self-hosted under the SIL Open Font License 1.1. The sun, moon and flower-of-life artwork was drawn in original SVG for this site.
Questions or corrections: contact the Decoders team.