These calculations are based on the Easter Table of Dionysius Exiguus. A year is input, and various relevant data is returned, including the calculation of Easter.
These calculations are in line with the works of Dionysius Exiguus, which means they are working of the old, Julian Calendar. Furthermore, the calculations of the past were largely done with the Orthodox calendar in mind. The date of Easter returned reflects that, returing the Orthodox date for Easter. If you wish to see a western calculation for Easter, please click here.