The Thai people count their years from when Buddha was born, which is year 0 for them. Buddha was born on 543 BC. So, when we say year 2005, the Thai people say year 2548.
Why was it year 2005 on the calendar when I was there in 2006? Old calendar? No, that’s because they celebrate their New Year in April month. So they didn't changed year yet when I was there in February month.