Buzz - They use the Julian Calendar; we use the Gregorian.
March 21, the assumed equinox is slowly slipping later than the astronomical equinox on the Julian Calendar. It is currently about two weeks later. The Gregorian corrected this by omitting leap year three times every four centuries. It also skipped several days to bring march 21 back to the astronomical equinox.
Because the Gregorian calendar was instituted by the pope we use it liturgically but the Orthodox and Jews don’t.
A search on Gregorian Calendar and/or Julian Calendar should give you more than any reasonable person would want to know.