Buddhist calendar is bit different from Gregorian Calendar. The main difference is the year in Buddhist calendar is 543 years more than the Gregorian calendar.
The concept: Gregorian calendar starts with the birth of Christ and anything before that will consider or label as BC.
Buddhist calendar starts with death of Buddha. Even the concept is very specific, there are various versions of Buddhist calendar available.
According to Thai Buddhist calendar, Buddha passed away 543 years before Jesus Christ. So Thai Calendar is just 543 years + Christian Calendar.
If the date is 01-Jan-2016, then Thai date will be 01-Jan-2559.
User can choose in both calendar. If you select the date in normal calendar, it automatically converts in to Thai format and vise versa.
This will be available for all date and datetime formats.