The Eastern Orthodox Churches are evidently individuated by states/nations, being most prominent in eastern Europe and Russia. The autonomy of statehood seems reflected also in some of the practices of state churches, the calendar being a rather divisive issue among them. A schism between Old and New Calendarists developed in this century, when in 1923 several state churches approximated the Gregorian Reform of 1582. The Russian and Greek churches were among those represented at an ecumenical council in which the reform proposal was approved. Technically, they did not fall into step with the Greogrian calendar, even though they skipped the requisite number of days to agree with it. They additionally adopted the new leap-year rule Chris Carrier has mentioned: Leap year every four years except years which leave remainders of 200 or 600 when divided by 900. (http://ecuvax.cis.ecu.edu/~pymccart/orthodox-reform.html) Not all state churches agreed to the reform, and some churches split internally over it. I have read that a subsequent compromise has been attempted, whereby the New Calendarists have readopted the Julian calendar for fixing moveable feasts related to Easter (the principal issue of contention), but observe other feasts, like Christmas, in agreement with the Gregorian calendar. As for the Orthodox church of Finnland, I have seen it mentioned in a few places as a lone exception, having adopted the Gregorian dates for all feasts. The explanation given is usually that the Christian population of Finnland is overwhelmingly Western in orientation, so the Orthodox in Finnland prefer to fit in. As for the FORMULA for computing Orthodox Easter, it would be the same as the Western formula, since both Churches regard the Council of Nicaea in 325 AD as authoritative. But because of the differences in the Julian and Gregorian calendars, the date of the Vernal Equinox (March 21) differs by about two weeks. There's another difference, too: relating to when/WHERE the moon is full. (http://cssa.stanford.ecu/~marcos/ortheast.html) --Rick McCarty