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. Cycle The Cambodian, Lao 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. The Cambodian calendar also maintains a 10-year naming cycle (numbered one to ten). Cambodians use multiple systems to identify a given year. For instance, 2017 is identified as 2561 Buddhist Era, the year of Rooster, Nuppasak (Year 9). The Thai lunar calendar also uses a similar numbered 10-year cycle. Each number in the cycle corresponds to the last digit of the year in the Chula Sakarat calendar. Accuracy 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. History 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