The calendars differ. The western calendar used to have ten months. Then two Roman emperors thought they were stars in heaven; they added their names, Julius and Augustus Caesar, so two months were added, July and August. But the Roman (Julian) computers were not programmed properly! They forgot to re-number the months. Sept means seven but September is the ninth month! Oct means eight but October is the tenth month! Nove means nine but November is the eleventh month! Dece means ten but December is the twelfth month! Even the Gregorian calendar did not make the necessary adjustments.