For the coinage of Thailand, three different dating systems and calendars are used. This has to do with the historical time periods that the coins were minted under for the use of the chosen system.
For the first machine-struck coinage of Thailand the Chula Sakarat or Chulasakarat system was used. This was a calendar that used both lunar and solar systems to define dating. The Chula Sakarat system was derived from the Burmese calendar, which itself was derived from a Hindu calendar system. The calendar system began in 640 Common Era (CE). Due to the differences in the Common Era and the Chula Sakarat system means to get the Common Era date simply add 638 to the Chula Sakarat (CS) date. This dating system is used on Thailand machine struck coins from under King Rama III with issues being dated CS1197 (1835).
The next dating system of Thailand is the Ratanakosindosk Era dating or RS. This system follows a calendar of the Thailand version of the Gregorian calendar. This system was intended to replace the Thailand lunar calendar. The year began with year 1 the accession date of King Rama I or 1782 Common Era. Under King Rama V, coins started to use the Ratanakosindosk or RS year system. To convert the date simply add 1781 to the RS year and you get the Common Era converted date.
The final dating system for coinage in Thailand is the Buddhist Era (BE) system. This lunisolar system is one where months are based on lunar cycles and years on solar years. The objective is to sync the lunar and the solar part of time. The year dating starts with Buddhist Era or BE year zero, which would equate to about 544 before Common Era dating. For the year 2020 Common Era, the BE year would be 2563. To get the Common Era date, simply subtract 543 from the BE year and you will get the conversion. This system started under King Rama VI and is still used today with Maha Vajiralongkorn King Rama X. Several modern coins issued by Thailand will have joint dates with both the BE year and the CE year but the vast majority only use the BE date. There are a few examples of international commemorative issues that will only use the common era date on the coin.
In the last few years, PCGS has worked hard to improve and update the dating on the label. For the coins of Thailand, PCGS will note both years with the Common Era along with the given dating system date featured on the coin.