DATES
Calendars:A calendar is a system for organising days into culturally meaningful units (weeks, months, years). Earth's six primary calendars in active civil or religious use today are Gregorian, Julian, Hebrew, Islamic, Indian, and Chinese.
All dates below are computed from the same UTC instant. Lunar and lunisolar dates may roll at sunset rather than midnight — treat ±1 day as expected near the boundary.
The same instant in time, expressed in every calendar humanity still uses. Julian Day Number (JDN) is the absolute day count shared by every system.
Six Primary Calendars
Introduced 1582 by Pope Gregory XIII. The international civil standard, with a 365.2425-day mean year.
Introduced 45 BCE by Julius Caesar. Mean year 365.25 days; still used by parts of the Eastern Orthodox Church.
12 lunar months synchronized to the solar year by 7 leap months in every 19-year Metonic cycle. Epoch: 3761 BCE.
Pure lunar calendar of 12 months × 29–30 days ≈ 354 days/year. Epoch: 622 CE (the Hijra).
India's national civil calendar (1957 reform of the Saka era). Months tied to the tropical year.
Lunar months synchronized to solar terms. Used to determine traditional festivals across East Asia.
Other Calendars in Use
Iran's official calendar — astronomically the most accurate solar calendar in use.
Used in Thailand, Cambodia, Laos. Year = Gregorian + 543 (Buddha's parinirvana epoch).
12 months × 30 days + a 5- or 6-day intercalary month. ~7–8 years behind Gregorian.
Egyptian Christian calendar. Identical structure to the Ethiopian; epoch 284 CE (Era of Martyrs).
Mesoamerican day-count starting 11 Aug 3114 BCE. Format baktun.katun.tun.winal.kin.
ISO standard YYYY-Www-D. Week 1 contains the first Thursday of the year.
Hebrew, Islamic (civil tabular), Indian, Chinese, Persian, Buddhist, Ethiopian and Coptic conversions use the platform's Intl.DateTimeFormat calendar engine (ICU/CLDR). Julian and Maya Long Count are computed directly from the Julian Day Number.
Reference list: Wikipedia · List of calendars.
More calendars (Bahá'í, Igbo, Yoruba, traditional Korean, Vikram Samvat, Tibetan, etc.) will be added as the canonical conversion tables are folded in. The dashboard will surface a curated subset.