Reference

The seven calendar systems

UCP separates astronomical time from civil timekeeping. Each system below is locked to the same UTC instant, so none of them can drift away from the others.

Gregorian

Civil standard

The civil calendar used as the anchor every other system is compared against.

Xia (Hsia)

Solar terms and stems

Tracks the 24 solar terms and the pure mathematical progression of the sexagenary stem-branch cycle for year, month and day.

Chinese traditional

Lunisolar

The familiar lunar month and day used for festivals such as the Spring Festival.

Hindu lunisolar

Vedic, Lahiri ayanamsa

Tithi and nakshatra computed from the true astronomical positions of the Sun and Moon, using the Lahiri ayanamsa.

Buddhist Era

Civil era

The Theravada civil era, offset from the Gregorian year and used across Southeast Asia.

Saka Samvat

Tropical solar

India's national civil calendar: a tropical solar calendar with a fixed start on 22 March, or 21 March in a leap year.

Hijri

Umm al-Qura

The algorithmic Islamic calendar used as the official standard in Saudi Arabia.

Why calendars drift elsewhere

Most software converts calendars by tabulating offsets between them. Every intercalation adds a little error, and the systems slowly separate. UCP derives each calendar from the same astronomical instant instead, so the outputs cannot disagree about what day it is.

What this does not solveReligious observance sometimes depends on local sighting rather than calculation. UCP gives the astronomical and algorithmic answer; authorities may still rule differently.

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