The Thai calendar uses the Buddhist Era (BE), which is 543 years older than the Christian Era (AD). To convert from a BE date to an AD date, subtract 543. Thus BE 2543=2000. Before April 1, 1889 AD, Thailand used a lunar calendar of 12 or 13 months each with 29 or 30 days, each month starting with the new moon. The Gregorian calendar was adopted on April 1, 1889.