Adoption of the Gregorian calendar for secular use occurred in Eastern Orthodox countries as late as the 20th century and some religious groups in some of these countries still use the "old style" (O.S.) Julian calendar for ecclesiastical purposes. The Kingdom of Bulgaria changed from the Julian to the Gregorian calendar during the First World War on 31 March 1916, where the next day was 14 April 1916. The Ottoman Empire's Rumi calendar, used for fiscal purposes, was realigned from a Julian to a Gregorian starting on 16 February/ 1 March 1917. The beginning of the year was reset to 1 January starting in 1918.
Adoption of the Gregorian calendar decree expressly limited the reform to lay (i.e. non-religious) matters, as did the Greek decree. None of these reforms affected the dates of religious holidays. (See below.) While the civil administrations of eastern European countries adopted the Gregorian calendar in the 1910s or early 1920s, none of the national Eastern Orthodox Churches have recognised the Gregorian calendar for church or religious purposes. Instead, a Revised Julian calendar was proposed in May 1923 at the . It uses a different leap year rule, modifying a proposal of 1785 in such a way as to maximise the time before its dates start