The calendar has been used continuously in various Burmese states since its purported launch in 640 CE in the Sri Ksetra Kingdom, also called the Pyu era.
Scholarship accepts the chronicle narrative regarding the North Indian origin of the calendar and the chronology of adoption in Burma up to the Mahāsakaraj Era.
Recent research suggests that the Gupta Era (epochal year of 320 CE) may also have been in use in the Pyu states.
[note 1] Mainstream scholarship, however, holds that the recalibrated calendar was launched at Sri Ksetra, and later adopted by the upstart principality of Pagan.
[8][note 2] However, scholarship says the earliest evidence of Burmese calendar in modern Thailand dates only to the mid-13th century.
[9] While the use of the calendar appears to have spread southward to Sukhothai and eastward to Laotian states in the following centuries,[8] the official adoption farther south by the Ayutthaya Kingdom and farther east by Lan Xang came only after King Bayinnaung's conquests of those kingdoms in the 16th century.
[10][11] The Siamese adoption turned out to be the main catalyst for the calendar's usage in Cambodia,[12] a periodic vassal of Siam between the 16th and 19th centuries.
Likewise, the calendar spread to the Chittagong region of Bengal, which was dominated by the Arakanese Mrauk-U Kingdom from the 15th to 17th centuries.
[14] Furthermore, the Burmese system did not incorporate advances in Indian calculation methods of the sidereal year until the mid-19th century.
Meanwhile, the growing cumulative discrepancy between the civil solar and luni-solar years attracted increasing attention.
The present Surya Siddhanta (i.e., Saura school) was introduced to the Konbaung court in 1786, and was translated into Burmese after about 50 years.
[13][18] Finally, a new system called Thandeikta was proposed by Nyaunggan Sayadaw, a Buddhist monk, in Year 1200 (1838 CE).
The prevailing Metonic schedule was modified, and intercalary months were so fixed as to prevent further divergence between the solar and luni-solar years.
With the support of Princess Sekkya Dewi, who later became the chief queen of King Mindon, the new system was fully adopted in 1853.
The calendarists have periodically resorted to modifying its intercalation schedule, based on apparent reckoning, to keep pace, at the expense of making publishing future calendars more than a few years out all but impossible.
The calendar fell out of official status in several mainland Southeast Asian kingdoms (except Burma) in the second half of the 19th century with the arrival of European colonialism.
Thailand has moved on to its own version of the Buddhist calendar since 1941, although the Chulasakarat era dates remain the most commonly used and preferred form of entry in academic use for historical studies.
[22] Although the popular usage never extended beyond baho and nayi measurements, the calendar consists of time units down to the millisecond level.
Only the following are used in calendrical calculations: Therefore, modern time units can be expressed as: The civil week consists of seven days.
[24] The Synodic months are used to compose the years while the 27 lunar sidereal days (နက္ခတ် [nɛʔkʰaʔ]; from Sanskrit nakshatra), alongside the 12 signs of the zodiac, are used for astrological calculations.
The three exceptions—Mleta/Myweta (မ္လယ်တာ / မြွယ်တာ), Nanka (နံကာ), Thantu (သန်တူ)—which all fall during the Buddhist Lent, have been replaced by newer Burmese names (Waso, Wagaung, Thadingyut), which used to mean just the Full Moon days of the three months.
[24] In the Arakanese calendar, the month of Tagu gets the extra intercalary day in great leap years.
The average length of the month is further corrected by adding a day to Nayon at irregular intervals—a little more than seven times in two cycles (39 years).
If the remainder tallies with the set sequence number of the prevailing Metonic cycle, then it will be an intercalary year.
The calendar used to employ a 12-year Jovian cycle that redeployed the lunar month names and attached them to the years.
[note 9] Burmese calendarists have dealt with the issue by using apparent reckoning and periodically modifying the intercalation schedule in the Metonic cycle.
Thus each zodiac day, called nekkhat, represents a lunar mansion, or a segment of the ecliptic along which the Moon revolves around the Earth.
According to Arakanese (Rakhine) tradition, the calendar was launched by King Thuriya Thehta of the Dhanyawaddy dynasty.
In the Arakanese calendar, the month of Tagu gets the extra intercalary day in great leap years.
[37]) Likewise, Cambodian and Thai systems have retained the practice of giving animal names to the years from a cycle of 12.