Buddhist calendar

The Buddhist calendar is used on mainland in the countries of, , ,  (formerly ) and  in several related forms. It is a having months that are alternately 29 and 30 days, with an intercalated day and a 30-day month added at regular intervals. All of its forms are based on the original , not its modern form (both forms are used by the various s).

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 and is substantially longer than the modern. 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.

The month names are (except in old Burmese):
 * Caitra, Vaisakha, Jyestha, Ashadha, Sravan, Bhadrapada,
 * Asvina, Karttika, Margasirsha, Pausha, Magha, Phalguna.

The old month names were:
 * Tagu, Kason, Nayon, Waso, Wagaung, Tawthalin,
 * Thadingyut, Tarzaungmon, Natdaw, Pyadho, Tabodwe, Tabaung.

Common years have months that alternate 29 and 30 days with an extra day being added to Jyestha/Nayon making it 30 days, and an extra month is obtained by counting Ashadha/Waso twice. Each month has a waxing half of 15 days and a waning half of 14 or 15 days.

Kason, Nayon, First Waso, and Second Waso have 30 days each and are called the "four even continious months" in a year with an extra month.

The numbered year coincides with the sidereal year containing twelve al signs (rasi) so it can begin on any date from 6 Caitra/Tagu to 5 Vaisakha/Kason, meaning the rest of the month will be in an adjacent year. Thus any particular numbered year may be missing some days of the month while an adjacent year has the same set of dates at both its beginning and end.

Four eras were/are used: Anchansakarat (from ) (rarely used), Buddhasakarat (Buddhist Era or BE,  ) (BE–AD of 544 used to be common, but BE–AD is now 543 in Thailand, beginning after April before, then began and still begins ), Mahasakarat  (same as the Saka Era in India, used in Thailand until the mid-, standard in Cambodia), and Chulasakarat  (adopted in Thailand mid-13th century, standard in Burma). All years are elapsed/expired/complete years, thus their epochal year is, not year 1, because a complete year had not yet elapsed during it. The epochal dates only apply to year 0 &mdash; modern dates for the entry of the Sun into the first rasi (the beginning of the sidereal year) occur later in the due to. The calculations do not begin with zero at epoch &mdash; instead an offset of a certain number of whole and fractional days, which can amount to more than one year, must be added to all calculations, explaining the apparent Buddhasakarat inconsistency. Here 544 has an offset of 4 days at epoch whereas 543 has an offset of 369 days.