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.
Leap week calendar full weeks (364 or 371 days). Leap week calendars vary on whether the concept of month is preserved and whether the month (if preserved) has a whole number of weeks. The Pax Calendar and Hanke-Henry Permanent Calendar (formerly the Common-Civil-Calendar-and-Time) preserve or modify the Gregorian month structure. The ISO week date and the Weekdate Dating System are examples of leap week calendars that eliminate the month. Most leap week calendars take advantage of the 400-year cycle of the Gregorian calendar, which has exactly 20,871 weeks. With 329 common years of 52 weeks plus 71 leap years of 53 weeks, leap