The 1923 Congress and the Milankovic Leap Rule

In May 1923 a pan-Orthodox congress meeting in Constantinople took up a proposal to bring the fixed feasts back into alignment with the seasons without simply adopting the Gregorian calendar wholesale. The congress's astronomical delegate, the Serbian scientist Milutin Milankovic, proposed a new leap year rule: a year divisible by four is a leap year, except that a year divisible by one hundred is not, unless dividing it by nine hundred leaves a remainder of two hundred or six hundred, in which case it is a leap year after all. That rule tracks the true solar year even more closely than the Gregorian reform of 1582, and the two calendars will not diverge by even a single day until the year 2800. On the ground the change was simple: 1 October under the old Julian reckoning became 14 October, dropping thirteen days at a stroke. What the congress adopted was narrower than a full modernisation. It moved only the fixed feasts onto the new reckoning; the moveable cycle governing the date of Pascha stayed on the unrevised Julian computation, so churches that took up the reform kept a different Christmas from their Old Calendarist neighbours while continuing to celebrate Easter on exactly the same day as they always had.