Contemporary calendar has began its existence in the cristian era, in 526, started by pope John I. The chronologist of the pope, Dionysius Exiguus, worked further on the calendar, especially concerning Easter.
During the Middle Ages some problems were dscovered with the use of the Julian calendar: every century had 3 to 4 days to much days. In the sixteenth century the mistake reached 10 days. It is the reason of calendar reformation in 1582, by the pope Gregorius XIII. He decided that 4 October should be followed by 15 Oktober at once. Also he decided that all of the leapyears of the full century years, which were not dividable by 400 would fall off. At this manner 1900 was not a leapyear, 2000 was a leapyear and 2100 will not.
The average duration of the Gregorian year is so 365.2425 days. The diffirence with the real tropical year is so small, that a new reformation will be needed in very, very far future.
The Gregorian calendar was worked on by the Calabrian medicician Aloysius Lilius, as well as by the papal commissioner C.Clavius, before it has reached the modern perfection.
The new calendar got in use very slowly:
- England and colonies (Northern America) in 1752; - whole Germany in 1776; - Sweden 1823; - Russia in 1918 (the difference with Julian calendar was already 13 days!) - In The Netherlands the calendar took its start at diffirent stages, in diffirent provinces.