The dates shown in twin's tombstone refer to the Thai Calendar, which is based on the birth of Buddha and have a 543 years difference to occidental calendar. The sisters have born in 2518 BE what corresponds to 1975 AD; Pim has died in 2533 BE (1990 AD). The events depicted in the movie took place in 2549 BE (2006 AD) since that's Ploy's date of death.