Revised Julian calendar

The Revised Julian calendar, also known as the Milanković calendar, or, less formally, New calendar, is a calendar, developed and proposed by the Serbian scientist Milutin Milanković in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar that has come to predominate worldwide. This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church. The Revised Julian calendar temporarily aligned its dates with the Gregorian calendar proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world.

The new calendar was proposed for adoption by the Orthodox churches at the Pan-Orthodox Congress of Constantinople () in May 1923 and subsequently it was adopted by several of the Orthodox churches. The synod was chaired by controversial Patriarch Meletius IV of Constantinople, and called Pan-Orthodox by its supporters. But only the Patriarch of Constantinople and the Serbian Patriarch were represented. There were no representatives of the other members of the original Orthodox Pentarchy (the Patriarchates of Jerusalem, Antioch, and Alexandria) or from the largest Orthodox Church, the Russian Orthodox Church.

This synod synchronized the new calendar with the Gregorian calendar by specifying that the next 1 October of the Julian calendar would be 14 October in the new calendar, thus dropping thirteen days. It then adopted the leap rule of Milanković, an astronomical delegate to the synod representing the Kingdom of Serbs, Croats and Slovenes. Milanković selected this rule because its mean year was within two seconds of the then current length of the mean tropical year. The present vernal equinox year, however, is about 12 seconds longer, in terms of mean solar days.

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