- The Gregorian Calendar, a calendar introduced in 1582 by Pope Gregory XIII of the Roman Catholic Church and used in the Churches of Finland and Estonia.
- The Revised Julian Calendar, the calendar considered by a synod of Orthodox churches in 1923 in Constantinople. This is functionally identical to the Gregorian Calendar except for the Paschal cycle (which is still reckoned according to the Julian timetables) and leap year calculation.
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