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 and 1910, and again from 1948 to 1962, Korean era names were used for its years; and between 1910 and 1945, when Korea was under Japanese rule, Japanese era names were used to count the years of the Gregorian calendar used in Korea. In South Korea, from 1945 until 1961, Gregorian calendar years were also counted from the foundation of Gojoseon in 2333 BC (regarded as year one), the date of the legendary founding of Korea by Dangun, hence these Dangi (단기) years were 4278 to 4294. This numbering was informally used with the Korean lunar calendar before 1945 but