The Buddhist calendar is a set of lunisolar calendars primarily used in mainland Southeast Asian countries of Cambodia, Laos, Myanmar and Thailand as well as in Sri Lanka and Chinese populations of Malaysia and Singapore for religious or official occasions. While the calendars share a common lineage, they also have minor but important variations such as intercalation schedules, month names and numbering, use of cycles, etc. In Thailand, the name Buddhist Era is a year numbering system shared by the traditional Thai lunisolar calendar and by the Thai solar calendar.
The Southeast Asian lunisolar calendars are largely based on an older version of the Hindu calendar, which uses the sidereal year as the solar year. One major difference is that the Southeast Asian systems, unlike their Indian cousins, do not use apparent reckoning to stay in sync with the sidereal year. Instead, they employ their versions of the Metonic cycle. However, since the Metonic cycle is not very accurate for sidereal years, the Southeast Asian calendar is slowly drifting out of sync with the sidereal, approximately one day every 100 years. Yet no coordinated structural reforms of the lunisolar calendar have been undertaken.
Today, the traditional Buddhist lunisolar calendar is used mainly for Theravada Buddhist festivals, and no longer has the official calendar status anywhere. The Thai Buddhist Era, a renumbered Gregorian calendar, is the official calendar in Thailand.
The calculation methodology of the current versions of Southeast Asian Buddhist calendars is largely based on that of the Burmese calendar, which was in use in various Southeast Asian kingdoms down to the 19th century under the names of Chula Sakarat and Jolak Sakaraj. The Burmese calendar in turn was based on the "original" Surya Siddhanta system of ancient India (believed to be Ardharatrika school). One key difference with Indian systems is that the Burmese system has followed a variation of the Metonic cycle. It is unclear from where, when or how the Metonic system was introduced; hypotheses range from China to Europe. The Burmese system, and indeed the Southeast Asian systems, thus use a "strange" combination of sidereal years from Indian calendar in combination with the Metonic cycle better for tropical years.
In all Theravada traditions, the calendar's epochal year 0 date was the day in which the Buddha attained parinibbāna . However, not all traditions agree on when it actually took place. In Burmese Buddhist tradition, it was 13 May 544 BCE (Tuesday, Full moon of Kason 148 Anjanasakaraj). But in Thailand, it was 11 March 545 BCE, the date which the current Thai lunisolar and solar calendars use as the epochal date. Yet, the Thai calendars for some reason have fixed the difference between their Buddhist Era (BE) numbering and the Christian/Common Era (CE) numbering at 543, which points to an epochal year of 544 BCE, not 545 BCE. In Myanmar, the difference between BE and CE can be 543 or 544 for CE dates, and 544 or 543 for BCE dates, depending on the month of the Buddhist Era (as the Buddhist calendar straddles the Gregorian calendar—currently from April to April). In Sri Lanka, the difference between BE and CE is 544. 
|BE Year||Equivalent CE Years||Equivalent CE Year (Thai Solar)|
|543||1 BCE – 1 CE|
|544||1–2 CE||1–2 CE|
The calendar recognizes two types of months: synodic month and sidereal month. The Synodic months are used to compose the years while the 27 lunar sidereal days (Sanskrit: nakshatra ), alongside the 12 signs of the zodiac, are used for astrological calculations. (The Burmese calendar also recognizes a solar month called Thuriya Matha , which is defined as 1/12th of a year. But the solar month varies by the type of year such as tropical year, sidereal year, etc.)
Waxing and waning
The days of the month are counted in two halves, waxing and waning. The 15th of the waxing is the civil full moon day. The civil new moon day is the last day of the month (14th or 15th waning). Because of the inaccuracy of the calendrical calculation systems, the mean and real (true) New Moons rarely coincide. The mean New Moon often precedes the real New Moon.
|Waxing||1 to 15||from New Moon to Full Moon|
|Full Moon||15||Full Moon|
|Waning||1 to 14 or 15||from Full Moon to New Moon|
|New Moon||15||New Moon|
Number of days per month
As the Synodic lunar month is approximately 29.5 days, the calendar uses alternating months of 29 and 30 days.
|Sanskrit||Pali||Burmese||Khmer||Lao||Sinhala||Thai||# of days||~Gregorian|
|Chaitra||Citta||Tagu ( တန်ခူး )|| ចេត្រ |
|ຈິຕ||Bak (බක්)||Chittra (จิตร)||29||March–April|
|Vaisākha||Visakha||Kason ( ကဆုန် )|| ពិសាខ |
|ວິສາຂະ||Vesak (වෙසක්)||Wisakha (วิสาข)||30||April–May|
|Jyaiṣṭha||Jeṭṭha||Nayon ( နယုန် )|| ជេស្ឋ |
|ເຊດ||Poson (පොසොන්)||Chettha (เชษฐ)||29 ||May–June|
|Āṣāḍha||Āsāḷha||Waso ( ဝါဆို )|| អាសាឍ |
|ອາສາລະຫະ||Æsala (ඇසළ)||Asanha (อาสาฬห)||30||June–July|
|Śrāvaṇa||Sāvaṇa||Wagaung ( ဝါခေါင် )|| ស្រាពណ៍ |
|ສາວະນະ||Nikini (නිකිණි)||Sawana (สาวน)||29||July–August|
|Bhādrapada||Poṭṭhapāda||Tawthalin ( တော်သလင်း )|| ភទ្របទ |
|ພັດທະຣະບົດ||Binara (බිනර)||Phatthrabot (ภัทรบท)||30||August–September|
|Āśvina||Assayuja||Thadingyut ( သီတင်းကျွတ် )|| អស្សុជ |
|ອັດສະວະຍຸດ||Wap (වප්)||Atsawayut (อัศวยุช)||29||September–October|
|Kārttika||Kattikā||Tazaungmon ( တန်ဆောင်မုန်း )|| កត្តិក |
|ກັດຕິກາ||Il (ඉල්)||Kattika (กัตติกา)||30||October–November|
|Mārgaśirṣa||Māgasira||Nadaw ( နတ်တော် )|| មិគសិរ |
|ມິຄະສິນ||Undhuvap (උඳුවප්)||Mikkhasira (มิคสิร)||29||November–December|
|Pauṣa||Phussa||Pyatho ( ပြာသို )|| បុស្ស |
|ປຸສສ||Dhuruthu (දුරුතු)||Putsa (ปุสส)||30||December–January|
|Māgha||Māgha||Tabodwe ( တပို့တွဲ )|| មាឃ |
|ມາດ||Navam (නවම්)||Makha (มาฆ)||29||January–February|
|Phālguna||Phagguṇa||Tabaung ( တပေါင်း )|| ផល្គុន |
|ຜັກຄຸນ||Mædhin (මැදින්)||Phakkhun (ผัคคุณ)||30||February–March|
Various regional versions of Chula Sakarat/Burmese calendar existed across various regions of mainland Southeast Asia. Unlike Burmese systems, Kengtung, Lan Na, Lan Xang and Sukhothai systems refer to the months by numbers, not by names. This means reading ancient texts and inscriptions in Thailand requires constant vigilance, not just in making sure one is correctly operating for the correct region, but also for variations within regions itself when incursions cause a variation in practice. However, Cambodian (Khmer) month system, which begins with Margasirsa as the first month, demonstrated precisely by the names and numbers. 
|Month||Khmer, Lan Xang, Sukhothai and Old Burmese||Kengtung||Chiang Mai|
The Buddhist calendar is a lunisolar calendar in which the months are based on lunar months and years are based on solar years. One of its primary objectives is to synchronize the lunar part with the solar part. The lunar months, normally twelve of them, consist alternately of 29 days and 30 days, such that a normal lunar year will contain 354 days, as opposed to the solar year of ~365.25 days. Therefore, some form of addition to the lunar year (of intercalation) is necessary. The overall basis for it is provided by cycles of 57 years. Eleven extra days are inserted in every 57 years, and seven extra months of 30 days are inserted in every 19 years (21 months in 57 years). This provides 20819 complete days to both calendars. This 57-year cycle would provide a mean year of about 365.2456 days and a mean month of about 29.530496 days, if not corrected.
As such, the calendar adds an intercalary month in leap years and sometimes also an intercalary day in great leap years. The intercalary month not only corrects the length of the year but also corrects the accumulating error of the month to extent of half a day. 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). The intercalary day is never inserted except in a year which has an intercalary month. The Hindu calendar inserts an intercalary month at any time of year as soon as the accumulated fractions amount to one month. The Burmese calendar however always inserts the intercalary month at the same time of the year, after the summer solstice while the Arakanese calendar inserts it after the vernal equinox.
The actual Burmese calendar year consists of 354, 384 or 385 days.
|Month||Regular year||Small leap year||Big leap year|
Note: The Arakanese calendar adds the intercalary day in Tagu, not in Nayon.
Cambodian, Lao and Thai
The Cambodian, Lao and Thai lunar calendars use a slightly different method to place the intercalary day. Instead of it in a leap year as in the Burmese system, the Thai system places it in a separate year. Thus, the Thai small leap year has 355 days while the Thai great leap year has 384 days.
|Month||Regular year||Small leap year||Big leap year|
New Year's Day
Since the main purpose of Buddhist calendar is to keep pace with the solar year, the new year is always marked by the solar year, which falls at the time when the Sun enters Aries. The date, which at the present falls on the 17th of April, has slowly drifted over the centuries. In the 20th century, the New Year's Day fell on April 15 or 16th but in the 17th century, it fell on April 9 or 10th. Thailand and Cambodia no longer use the traditional lunisolar calendar to mark the New Year's Day.
|Tradition||Date in 2013||Notes|
|Burmese||17 April||Varies; will keep on drifting away|
|Thai||13 April||Fixed to the solar calendar|
Year cycle system
Animal year cycle system
Cambodian and Thai systems give animal names to the years from a cycle of 12. The practice also existed in Burma in the Pagan period but later died out.
|5||Nāga / Dragon||រោង (Rorng)|
Ten-year cycle system
In Cambodia, there is the sequence of year, which numbered from one to ten, used to identify a particular year, called " Sak " ( Khmer: ស័ក) that means " era ". It begins with a prefix "Aek, tor, trei" (which means one, two, three respectively, using numerals that were derived from the Sanskrit language), then the suffix "Sak" is added to each prefix so as to form a word of the year. Cambodian use the Sak system to differentiate the twelve animal years that return every twelve year by including the ten-year system along with the animal year system. For instance, in 2017, the ten-year and animal year fall in the year of Rooster, Nuppasak (era 9), Buddhist Era of 2561 (based on Cambodian lunar calendar). 
|Numbers||Names in Khmer||Transliteration|
The Southeast Asian Buddhist calendars use lunar months but try to keep pace with the solar year, by inserting intercalary months and days on the Metonic cycle (in the case of the Burmese calendar, on a modified Metonic cycle). However, the solar year as defined by the Buddhist calendars is really a sidereal year, which is nearly 24 minutes longer than the actual mean tropical year. Therefore, like all sidereal-based calendars, the lunisolar calendars are slowly drifting away from the seasons. The calendars are drifting one day approximately every 60 years and 4 months.
The accumulating drift against the seasons means the New Year's Day which used to fall on 22 March (near the vernal equinox) in 638 CE now falls on 17 April in 2013 CE. There is no known internationally concerted effort to stop this drift. Thailand has moved its "Buddhist Era" to the Gregorian calendar under the name of Thai solar calendar. In Myanmar, Burmese calendarists have tried to deal with the issue by periodically modifying the intercalation schedule in the Metonic cycle. One major downside of this approach is that it is not possible to publish future calendars more than a few years (often even a year) ahead.
The Buddhist Era was first introduced to Southeast Asia along with Buddhism in the early centuries CE. It was not a separate calendar but simply a year numbering system that employed the organization and calculation methods of the prevailing lunisolar calendars in use throughout the region. In the early centuries CE, the reference civil calendar of the Buddhist calendar prevalent in Southeast Asia was the Saka Era (Mahāsakaraj Era), which is said to have been adopted by the Pyu state of Sri Ksetra in 80 CE. The Saka Era was gradually replaced by the Burmese Era or Culāsakaraj, first in Myanmar in 640 CE, and in other Theravada kingdoms of Southeast Asia between the 13th and 16th centuries. Theravada Buddhist tradition also recognizes pre-Buddhist Anjana Sakaraj (Añjana's Era) since the events of the Buddha's life are recorded in that era.
|Anjana Sakaraj||10 March 691 BCE|| Said to have been started by the Buddha's maternal grandfather King Añjana |
Used to date the events during the Buddha's lifetime
|Buddhist Era|| 13 May 544 BCE |
11 March 545 BCE
|544 BCE in Myanmar; 545 BCE in Thailand|
|Saka Era||17 March 78 CE||Civil calendar|
|Burmese Era ( Culāsakaraj)||22 March 638||Civil calendar|
The tradition of using different reference calendars continued in Siam in 1912 when King Vajiravudh decreed that the Buddhist Era would now track the Thai solar calendar, the Siamese version of the Gregorian calendar with the New Year's Day of 1 April. Therefore, the Thai Buddhist Era year of 2455 began on 1 April 1912 (as opposed to 15 April 1912 according to the lunisolar calendar). The Thai Buddhist Era was further realigned to the Gregorian calendar on 6 September 1940 when Prime Minister Phibunsongkhram decreed 1 January 1941 as the start of the year 2484 BE. As a result, the Year 2483 was only 9 months long, and the Thai Buddhist Era equals that of the Common Era plus 543 years.
The lunisolar calendar is used to mark important Buddhist holidays. Many of the holidays are celebrated as public holidays.
|Buddhist calendar date||International date||Public holiday in||Notes|
|Full moon of Pausa||January||Sri Lanka||Duruthu Poya: Commemorates the first visit of the Buddha to Sri Lanka|
|Full moon of Magha||February||Cambodia, Laos, Sri Lanka, Thailand||Magha Puja in Cambodia, Laos, Thailand and known as Navam Poya in Sri Lanka|
|Full moon of Phalguna||March||Laos, Myanmar, Sri Lanka||Boun Pha Vet (Laos), Tabaung Festival (Myanmar), Medin Poya (Sri Lanka)|
|Almost always in Caitra, sometimes in Vaisakha|| 13–17 April |
(varies by country)
|Cambodia, Laos, Myanmar, Sri Lanka, Thailand|| Songkran (Southeast Asian New Year) |
Traditionally, the New Year's Day is marked when the Sun enters Aries but the day is now fixed in most countries; Myanmar still follows the tradition. It also marks the beginning of the next Buddhist calendar animal zodiac year for certain countries.
|Full moon of Caitra||April||Sri Lanka||Bak Poya: Commemorates the second visit of the Buddha to Sri Lanka|
|Full moon of Visakha||May||Cambodia, Laos, Thailand, Sri Lanka, Myanmar||Buddha Day ( Vesak)|
|Full moon of Jyaistha||June||Sri Lanka||Poson Poya: Commemorates introduction of Buddhism to Sri Lanka|
|Full moon of Ashadha||July||Cambodia, Laos, Myanmar, Thailand, Sri Lanka|| Vassa |
Esala Poya (Sri Lanka)
Asalha Puja (Thailand)
|Full moon of Sravana||August||Sri Lanka||Nikini Poya|
|Full moon of Bhadrapada||September||Laos, Sri Lanka||Binara Poya (Sri Lanka)|
|Full moon of Asvina||October||Cambodia, Laos, Myanmar, Sri Lanka|| End of Vassa |
Boun Suang Huea (Laos); Thadingyut Festival (Myanmar); Vap Poya (Sri Lanka); Wan Ok Phansa (Thailand)
|Full moon of Karttika||November||Laos, Myanmar, Sri Lanka, Thailand, Cambodia||That Luang Festival (Laos); Tazaungdaing Festival (Myanmar); Il Poya (Sri Lanka); Loi Krathong (Thailand); Bon Om Touk (Cambodia)|
|Full moon of Margasirsa||December||Sri Lanka||Undhuvap Poya (Sri Lanka)|
Since the release of Java 8, the Buddhist calendar is supported in the new Date and Time API.
- (Ohashi 2001: 398–399): Astronomers of ancient India certainly knew of the Metonic cycle, and may have introduced the concept to Southeast Asia. However, the Metonic cycle, which employs tropical years, is incompatible with sidereal based Hindu calendars, and thus was not (and still is not) used in Hindu calendars. Chatterjee (1998: 151) suggests that the Metonic system was introduced to Burma by Europeans. Ohashi (2001: 398–399) rejects Chatterjee's hypothesis saying that "no other trace of European influence is found in South-East Asian astronomy." Instead, Ohashi (2001: 401–403) suggests that China may have been the source of the Metonic cycle.
- (Irwin 1909: 26–27): In the mid-19th century, the Burmese Konbaung Dynasty tried to address the issue by introducing a new calculation methodology. However, the new solar year it chose was actually 0.56 second a year less accurate than the version still prevalent in the rest of Southeast Asia. The Konbaung court also modified the Metonic cycle, which did more to re-synchronize the calendar with the seasons than the less accurate solar year.
- (Eade 1989: 11): The earliest use of the Burmese calendar in lands part of present-day Thailand dates to the mid-13th century. (Smith 1966: 11): Ayutthaya adopted the Burmese calendar in the 16th century.