The Buddhist calendar is used on mainland Southeast Asia in the countries of Thailand, Cambodia, Laos, Burma (officially known as Myanmar) and Sri Lanka in several related forms. It is a lunisolar calendar having months that are alternately 29 and 30 days, with an intercalated day and a 30-day month added at regular intervals. All forms of the Buddhist calendar are based on the original version of the Surya Siddhanta, which dates to the 3rd century CE (both the original and medieval forms of the Surya Siddhanta are used by the various Hindu calendars).

Its lunisolar intercalation system generally adds seven extra months (adhikamasa) every 19 years and 11 extra days (adhikavara) every 57 years, but this is only a rough guide to the results of the actual calculations. The average year is 365.25875 days reckoned from the mahayuga of 4,320,000 years, simplified to 292,207 days every 800 years by removing a common factor of 5400 from the total days and years. This year is slightly longer than the modern sidereal year and is substantially longer than the modern tropical year. The Hindu version adds extra months and days (or removes months and days) as soon as the astronomical formulae require, whereas the southeast Asian versions delay their addition. The Thai/Lao/Cambodian version does not permit an extra day to occur within years having an extra month, whereas the Burmese/Sri Lankan version permits an extra day only in years having an extra month. Thus there are four types of lunisolar years, of 354, 355, 384, or 385 days. Even though the intercalation cycles imply a tropical year, the sidereal year that is actually used causes the 'cycles' to gradually shift throughout history.