Revised Julian Calendar
The Revised Julian Calendar is a calendar that was considered for adoption by several Orthodox churches at a congress in Constantinople in May 1923. The congress synchronized the new calendar with the Gregorian Calendar by specifying that October 1, 1923, in the Julian Calendar will be October 14 in the Revised Julian Calendar, thus dropping thirteen days. It then adopted a leap year rule that differs from that of the Gregorian calendar: Years evenly divisible by four are leap years, except that years evenly divisible by 100 are not leap years, unless they leave a remainder of 200 or 600 when divided by 900, in which case they are leap years. This means that the two calendars will first differ in 2800, which will be a leap year in the Gregorian Calendar, but a common year in the Revised Julian Calendar. This leap year rule was proposed by Milutin Milankovic, an astronomical delegate to the congress representing the governments of the Serbs, Croatians, and Slovenes.
Milankovic selected this rule, which produces an average year length of 365.242222… days, because it was within two seconds of the then current length of the mean tropical year. However, the vernal equinox year is slightly longer, so for a few thousand years the Revised Julian Calendar doesn't do as good a job as the Gregorian Calendar at keeping the vernal equinox on or close to March 21. But the length of a day is increasing by about 1.7 milliseconds per century (due to tidal acceleration), so the number of days per year decreases by about 0.0001 each millennium. This means that in the long run, the Revised Julian Calendar will also be inaccurate even if the mean tropical year is the basis.
The Revised Julian calendar was adopted by the Orthodox Churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, and Bulgaria (the last in 1963), called the New Calendarists. It was rejected by the Orthodox Churches of Jerusalem, Russia, Serbia, Ukraine, and Georgia, and the Old Calendarists. Despite the fact that the Russian Orthodox Church continues to use the Julian Calendar for both its fixed feasts and for Pascha, Milankovic stated that it had already adopted the new calendar by October 1923 (It must have repudiated its decision shortly thereafter.)
The congress also adopted an astronomical rule for Pascha: Pascha is the Sunday after the midnight-to-midnight day at the meridian of the Church of the Holy Sepulchre in Jerusalem during which the first full moon after the vernal equinox occurs. Although the instant of the full moon must occur after the instant of the vernal equinox, it may occur on the same day. If the full moon occurs on a Sunday, Easter is the following Sunday. However, all Orthodox churches rejected this rule and continue to use the Julian Calendar to determine the date of Pascha (except for the Churches of Finland and Estonia, which use the Gregorian Paschalion).