Military calendar dating
These tables were revised during the following few centuries resulting, eventually in the tables constructed by the 6th century Abbot of Scythia, Dionysius Exiguus.
Nonetheless, different means of calculations continued in use throughout the Christian world.
In a congress held in 1923, the Eastern Churches adopted a modified Gregorian Calendar and decided to set the date of Easter according to the astronomical full moon at the meridian of Jerusalem.
For example, in 1962 the astronomical full moon occurred on March 21, UT=7h 55m - about six hours after astronomical equinox.
The statement that Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox, is only an approximate statement of the actual ecclesiastical rules.
The full moon involved is not the astronomical Full Moon but an ecclesiastical moon (determined from tables) that keeps, more or less, in step with the astronomical full Moon.
There are three major differences between the ecclesiastical and the astronomical systems.
Inevitably, the date of Easter occasionally differs from a date that depends on the astronomical full moon and astronomical vernal equinox.