Buddhist calendar

The Buddhist calendar (Pali: Sāsanā Sakaraj; Burmese: သာသနာ သက္ကရာဇ်, [θàðənà θɛʔkəɹɪʔ]; Khmer: ពុទ្ធសករាជ; Sinhala: බුද්ධ වර්‍ෂ or සාසන වර්‍ෂ (Buddha Varsha or Sāsana Varsha)) 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 sidereal year. Instead, they employ their versions of the Metonic cycle. However, since the Metonic cycle is not very accurate for sidereal years, the Southeast Asian calendar is slowly drifting out of sync with the sidereal, approximately one day every 100 years. Yet no coordinated structural reforms of the lunisolar calendar have been undertaken.

...
Wikipedia