Calendar reform

The Roman emperor Julius Caesar introduced the Julian calendar in 45 BC. The nativity of Jesus Christ was taken as the basis of chronology (year 1) afterwards in 525 AD. Each fourth year of the Julian calendar (leap year) has 366 days. The calendar does not coincide with the astronomical year, and by the 16th century Easter had shifted in relation to the Spring Equinox.

In 1582 Pope Gregory XIII adjusted the Julian calendar. In the new Gregorian calendar, the years that can be divided by 100 are leap years only in cases when they can also be divided by 400. In order to correct the existing error, the calendar omitted 10 dates. In Catholic countries the Gregorian calendar was introduced in October 1582 – the 4th date was followed by the 15th. The protestants and the orthodox reformed their calendars later. Poland adopted the new calendar in 1582 and Sweden in 1753. In Russia the calendar reform was finally completed in early 1918 when everywhere in the country, including Estonia, 1 February was followed by 14th of February.
 
Today, the Julian calendar, still used by several orthodox churches, lags 13 days behind the Gregorian calendar. In Estonian history, the dates recorded according to the February 1918 Julian calendar or the so-called Old Calendar, have to be recalculated according to the Gregorian calendar that was at that time valid elsewhere in the world, adding 13 days to the 20th century dates; 12 days to the 19th century dates etc.

Various organisations (e.g. some Tartu student corporations) are still celebrating their anniversaries according to the old calendar. There is much confusion in history writing where the calendar differences are often forgotten, and in noting historical dates in calendars.

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