Buddhist Calendar - Intercalation System

Intercalation System

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.