The Buddhist calendar is luni-solar, a combination of lunar and solar calendars. History of Buddhism suggests that in original Surya Siddhanta calendar of third century, months vary between 29 and 30 and at regular intervals, an intercalated day and a 30-day month is added to it. People living in Southeast Asia including Cambodia, Laos, Thailand, and Myanmar use the original Buddhist calendar.
Luni solar intercalation system
Every 19 years, this calendar version has seven extra months (adhikamasa) and every 57 years there are 11 extra days (adhikavara). On an average, a year has 365.25875 days, judged from the mahayuga of 4,320,000 years, simplified to 292,207 days every 800 years by removing common factor of 5400 from the total days and years. This year is slightly longer than modern sidereal year and substantially longer than modern tropical year.
Hindu version : Buddhism in India has additions and deductions based on astronomical formulae
Southeast Asian : Delayed additions
Thai/Lao/Cambodian : Extra day will not be added in the year having an extra month.
Burmese : Extra day will be added in the years having an extra month
Each version of the calendar has 354, 355, 384, or 385 days respectively.
» In Sanskrit, months include Chaitra, Vaisakha, Jyestha, Ashadha, Shravan, Bhadrapada, Asvina, Kartik, Maragasirsha, Pausha, Magha and Phalguna.
» Old Burmese months were Tagu, Kason, Nayon, Wazo, Wagaung, Tawthalin, Thadingyut, Tarzaungmon, Natdaw, Pyatho, Tabodwe and Tabaung.
Common years have 29 - 30 day months with an extra day added to Jyeshtha/ Nayon, making it 30 days. Extra month is added by doubling the month, Ashadha/ Wazo. Each month has waxing and waning of half, about 15 days.
Overlapping the sidereal year, the numbered year includes twelve zodiacal signs that can begin on any date from 6 Caitra/Tagu to 5 Vaisakha/Kason. Thus numbered years may not have some days of the month; an adjacent year having the same set of dates at both its beginning and end.
Chronology of Buddhism in India
Four eras introduced in Buddhist calendars :
Rarely used Anchansakarat : (Since March 10, 691 BC)
Commonly used Buddhasakarat : (The Buddhist Era from March 11, 545 BC)
Mahasakarat : (March 17, 78 – during Saka Era in India) Used in Thailand till the mid 13th century and standard in Cambodia
Chulasakarat : (March 22, 638) Adopted in Thailand mid-13th century and standard in Burma
As all years have completed, their epochal year is year 0 not year 1, as a full year had not elapsed. For entry of the Sun into the first rasi (the beginning of the sidereal year), the epochal dates only apply to year 0, while modern dates occur later in the Gregorian calendar due to precession of the equinoxes. Calculations do not begin with zero at epoch but an offset of certain whole and fractional days, which can amount to more than one year. This must be added to all calculations, which explains the Buddhasakarat inconsistency. 544, here have an offset of 4 days at epoch whereas 543 have an offset of 369 days.