The mathematics behind the calendar most of the Orthodox world eventually adopted was Serbian work. Milutin Milankovic, the geophysicist and astronomer better known today for the orbital cycles that bear his name, represented the Kingdom of Serbs, Croats and Slovenes at the 1923 Constantinople congress and, after the delegation's own first proposal was turned down, drafted the leap year rule the congress adopted as the Revised Julian calendar.
The Serbian Orthodox Church never adopted it. Unlike Greece and Romania, which took up the new calendar for fixed feasts while keeping the old one for Easter, the Serbian church kept the unrevised Julian calendar for its whole liturgical year, and it still does. The refusal was not a rejection of Milankovic's astronomy, which nobody disputed, but of the idea that a national synod's decision could settle something belonging to the whole communion, the same objection raised in Greece and Romania and one the Serbian hierarchy, still rebuilding its own authority after Ottoman centuries and the disruptions of the twentieth century, was in no position to treat lightly.
The result is a small, specific irony this atlas can state plainly: the formula that eventually let most Orthodox Christians keep Christmas on the same day as their Catholic and Protestant neighbours was designed by a scientist whose own church, to this day, does not use it. Serbian Christmas is still kept by the old reckoning, thirteen days after the date the Revised Julian calendar Milankovic built now assigns it.