Siamese generally used two calendars, a sacred and a popular (
vulgar in the classical sense). The vulgar or
minor era (จุลศักราช, chula sakarat) was thought to have been instituted when the worship of Gautama was first introduced,
 and corresponds to the
traditional Burmese calendar (abbreviated ME or BE, the latter not to be confused with the abbreviation for the Buddhist Era, which is the sacred era.)
King Chulalongkorn decreed a change in vulgar reckoning to the
Rattanakosin Era (abbreviated RE) (รัตนโกสินทรศก, Rattanakosin Sok abbreviated ร.ศ. and R.S.) The
epoch (reference date) for Year 1 was 6 April 1782 with the accession of
Rama I, the foundation of the
Chakri Dynasty, and the founding of
Bangkok (Rattanakosin) as
capital in 106 RE, AD 1888.
In Thailand the sacred, or
Buddhist Era, is reckoned to have an epochal
year 0 from 11 March
543 BC, believed to be the
date of the death of
Gautama Buddha. King
Vajiravudh (Rama VI) changed year counting to this Buddhist Era (abbreviated BE) and moved the start of the year back to 1 April in 2455 BE, AD 1912. As there is no longer any reference to a vulgar or popular era, the Common Era may be presumed to have taken the place of the former.
New Year, the time at which a new
calendar year begins and the calendar's year count is incremented, originally coincided with the
date calculated for Songkran, when the Sun transits the constellation of
Aries, the first
astrological sign in the
Zodiac as reckoned by
sidereal astrology: thus the year commenced on 11 April 1822.
 As previously noted, Rama VI moved the start of the year back to 1 April in 2455 BE, AD 1912.
On 6 September 1940, Prime Minister
 1 January 1941 as the start of the year 2484 BE, so year 2483 BE had only nine months. To convert dates from 1 January to 31 March prior to that year, the number to add or subtract is 542; otherwise, it is 543. Example:
Today, both the
Common Era New Year's Day (1 January) and the traditional
Thai New Year (สงกรานต์, Songkran) celebrations (13–15 April) are public holidays in Thailand. In the
traditional Thai calendar, the change to the next
Chinese zodiacal animal occurs at Songkran (now fixed at 13 April.)
Thai Chinese communities in Thailand, however, the
Chinese calendar determines the day that a
Chinese New Year begins, and assumes the name of the next animal in the
twelve-year animal cycle.