Thailand may still be a developing country, but when it comes to years, the Western world is hopelessly behind Thailand. While we are in 2023, as I write this, they are in the year 2566 in Thailand.
In the past, before 1889, Thailand used a lunar calendar as the official calendar. Because the lunar calendar does not align with the seasons, they would add an extra day or month in some years. That meant lunar calendar years could last 354, 355, or 384 days.
In 1889, King Chulalongkorn (Rama V) introduced the Gregorian calendar in Thailand. However, there was a twist - the new year started on April 1st; furthermore, instead of following the Christian calendar with the birth of Jesus as the starting point, the Thai calendar begins at the introduction of the Buddhist era, set 543 years before the start of the Gregorian calendar. The beginning of the year was changed from April 1st to January 1st in 1941.
Thais like to party; thus, they celebrate the new year three times. January 1st, according to the Gregorian calendar, April 13-15, they celebrate the Buddhist new year Songkran, and, finally, because many Thais are of Chinese heritage, they celebrate the Chinese new year, which falls in January or February.
Thailand still uses the lunar calendar for religious purposes and other purposes. Thai birth certificates include both the date according to the Gregorian calendar and the date according to the lunar calendar.