Eastern Christianity bases its calculations on the Julian Calendar. Because of the thirteen-day difference between the calendars between 1900 and 2099, 21 March corresponds, during the 21st century, to 3 April in the Gregorian Calendar. Easter therefore varies between 4 April and 8 May in the Gregorian calendar (the Julian calendar is no longer used as the civil calendar of the countries where Eastern Christian traditions predominate). Also, because the Julian "full moon" is always several days after the astronomical full moon, the eastern Easter is often later, relative to the visible Moon's phases, than western Easter.