The Buddhist calendar Thai: พุทธศักราช, RTGS: phutthasakkarat, [pʰút.tʰá.sàk.kà.ràːt]) is a set of lunisolar calendars primarily used in mainland Southeast Asian countries of Cambodia, Laos, Myanmar and Thailand as well as in Sri Lanka for religious and/or official occasions. While the calendars share a common lineage, they also have minor but important variations such as intercalation schedules, month names and numbering, use of cycles, etc. In Thailand, the name Buddhist Era is a year numbering system shared by the traditional Thai lunisolar calendar and by the Thai solar calendar.
The Southeast Asian lunisolar calendars are largely based on an older version of the Hindu calendar, which uses the sidereal year as the solar year. One major difference is that the Southeast Asian systems, unlike their Indian cousins, do not use apparent reckoning to stay in sync with the tropical year. Instead, they employ their versions of the Metonic cycle. However, since the Metonic cycle is intended to keep pace with tropical years, not sidereal years, the Southeast Asian calendar is slowly drifting out of sync with the tropical year, approximately one day every 60 years. The traditional New Year's Day, which used to take place around the vernal equinox in the mid-7th century now falls on 17 April today. Yet no coordinated structural reforms of the lunisolar calendar have been undertaken.
Today, the traditional Buddhist lunisolar calendar is used mainly for Theravada Buddhist festivals, and no longer has the official calendar status anywhere. The Thai Buddhist Era, a renumbered Gregorian calendar, is the official calendar in Thailand.