Most folks who've spent any time in Thailand know that the Buddhist calendar is about 500 years ahead of the Gregorian calendar. 543 years, to be precise. As I've read more, though, I've come across other systems of counting the years, all dating back to some important cultural event. Since I tend to get them confused, I decided to write a roundup of all the eras used in Thai:
Buddhist era (พ.ศ., พุทธศักราช or พุทธศก): Begins 543 B.C., the year the Buddha achieved nirvana. Traditionally a lunar calendar, Rama V adopted a solar calendar in 1888 A.D. The Thai Buddhist year also traditionally began at the arrival of the zodiac Aries, between April 13-15 on the modern calendar (still used to determine Songkran Day each year). However, in 1941, Field Marshal Plaek Phibunsongkhram, then Prime Minister, decreed January 1 of that year the first day of the Buddhist year 2484. This caused the year 2483 to have only 9 months (April-December).
Christian era or Common era (ค.ศ., คริสต์ศักราช, or คริสต์ศก): Begins 1 A.D., the year of Jesus Christ's birth according to tradition.
Muslim era or Hijra era (ฮ.ศ. or ฮิจญ์เราะหฺศักราช): Begins in 622, the year of the Hijra, when Mohammad emigrated from Mecca to Medina. This is a lunar calendar, with around 354 days per year, making the year number advance slightly faster than solar or lunisolar calendars.
Major era, Saka era or Shalivahana era (ม.ศ. or มหาศักราช): Begins 78 A.D., the year the Saka tribe defeated King Vikramaditya of Ujjain in northwestern India. Interestingly, sources seem to indicate that this is also the origin of the term ศักราช "era" in Thai. The word was formerly spelled ศกราช [สะกะหราด], with ศก or ศกะ referring to the Saka tribe. Thus, ศักราช literally means "Saka king", referring when the Sakas became kings of Ujjain. From there it has become a generic term for era, and nowadays the word ศก "sok" is also used in Thai to mean simply "year" (e.g. ศกนี้ "this year").
Minor era (จ.ศ. or จุลศักราช): Begins 638 A.D. This system was formerly believed to have been a Burmese system adopted by Thais, but since the first Burmese empire of Pagan wasn't established until 849, it is now believed only to have been later adopted by the Burmese. There are a few competing theories for what exactly this date marks, but it may well be the establishment of Sri Ksetra, capital of the Vikrama dynasty of the Pyu empire.
Bangkok era or Rattanakosin era (ร.ศ. or รัตนโกสินทรศก): Begins 1781 A.D., the year of the establishment of กรุงเทพฯ/Bangkok as the capital of เมืองไทย/Siam (Rattanakosin is another name for Bangkok, clipped from the city's extremely long ceremonial name). Construction of the city was finished the next year in 1782, which is the year from which anniversary celebrations of Bangkok are sometimes calculated. Rama V introduced this system as the first solar calendar in 1888.
In researching this post I even learned about a couple of very rare systems I'd never heard about (I've transcribed the names because I can't find standard English equivalents):
Kali yuga sakarat (กลียุคศักราช): Begins 3101 B.C., which is calculated to be the beginning of the current kali yuga ("age of vice"), according to the traditional Hindu four-stage yuga system.
Anchana sakarat (อัญชนะศักราช): Begins 690 B.C., established by King Anchana. I can't really find any more information about who this guy is.
So if you find someone asking you what year it is, you can tell them that this year is 5109, 2698, 2551, 2008, 1930, 1429, 1370, or 227. It all depends on who's counting.