The Thai solar, or Suriyakati (สุริยคติ), calendar is used in traditional and official contexts in Thailand, although the Western calendar is sometimes used in business.

The months and days of the week are the same as those used in the western Gregorian calendar, only their names differ. The year, however, is counted from the Buddhist Era (B.E.), which is 543 years earlier than the Christian Era (A.D.). For example, 2004 A.D. is equivalent to 2547 B.E. The era is based on the death of Gautama Buddha, which is dated to 543 BC by the Thai. (It is important to remember that only from January 1, 1941 onwards does this 543 addition/subtraction rule work perfectly. See below)

Until 1888 Thailand used a lunar calendar, in which the date of the New Year (Songkran) was between April 13 and 15. The new calendar, decreed by King Chulalongkorn (Rama V), was called Ratana Kosindra Sok, and was nearly identical with the western Gregorian calendar. Year counting, however, was in reference of the date of the founding of Bangkok (Ratana Kosindra), April 6 1782 (the first day of Year 1 Ratana Kosindra Era). King Vajiravudh (Rama VI) changed the year counting to Buddhist Era in 1912 and fixed the start of a year to April 1 (not sure who actually did the latter).

In 1941 (2484 B.E.), per decree by Prime Minister Phibunsongkhram, January 1 became the official start of a new year (so year 2483 B.E. had only nine months). When converting a date prior to that year, one should check whether it falls between January 1 and March 31: if so the number to add or subtract is 542, not 543.

Today, both New Year's Day (January 1) and Songkran (April 13-15) are public holidays. The buddhist feasts are still calculated according to the lunar calendar, so their dates change in the solar calendar every year.

See also

• Buddhist calendar