In May 1923 the Ecumenical Patriarch Meletius IV, born Emmanouil Metaxakis, convened a Pan Orthodox Congress in Constantinople with calendar reform high on its agenda. By the twentieth century the Julian calendar the church still kept had drifted thirteen days behind the seasons it was built to track. The congress was never attended by Alexandria, Antioch, Jerusalem or most other local churches, so nothing it resolved could bind the whole communion, only recommend to it.
The technical work fell to Milutin Milankovic, the Serbian geophysicist and astronomer serving as an astronomical delegate for the Kingdom of Serbs, Croats and Slovenes. When the congress rejected the Serbian delegation's first proposal, Milankovic drafted a second, and the leap year rule he produced, one that keeps the calendar aligned with the Gregorian reckoning for eight hundred and seventy seven years, until the year 2800, is what the congress adopted that October under the name the Revised Julian calendar.
What followed was not one reform but a slow, uneven sorting that is still visible today. Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, Albania and Bulgaria took up the Revised Julian calendar for fixed feast days over the following decades, keeping the older Julian reckoning for Easter. Jerusalem, Russia, Serbia and Georgia declined it outright. The boundary that resulted runs through the same body this atlas already describes as a communion without a single centre, and it tracks neither doctrine nor jurisdiction, only which calendar a given church's own faithful wake up to each December.