Gregorian Calendar

Invention

Worse, the reckoned Moon that was used to compute Easter was fixed to the Julian year by a 19-year cycle. However, that is an approximation that built up an error of 1 day every 310 years. So by the 16th century the lunar calendar was way out of sync with the real Moon too.

The fix for the equinox was to define that years divisible by 100 will be leap years only if they are divisible by 400 as well. So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. That gives a correction of 3 days. (Additionally, 10 dates were deleted at the time of introduction; see below.) This 3rd millennium will have 8 days corrected.

The Gregorian calendar also fixed the first day of the year as January 1, which was already the first day used in Italy, Germany, and other places, but not universally (England, for example, began the year on March 25). On January 1, 1622 January 1 was declared as the first day of the year.

When the new calendar was put in use, to correct the error already accumulated in the thirteen centuries since the council of Nicaea, a deletion of ten dates was made passing from October 4, 1582 directly to October 15, 1582. This created some consternation, and the church was accused of stealing days of people's lives.

The 19-year cycle used for the lunar calendar was also to be corrected by 1 day every 300 or 400 years (8 times in 2500 years). In fact, a new method for computing the date of Easter was introduced.

Denmark-Norway and the Protestantic parts of Germany adopted in 1700, due to the influence of Ole Rømer.

Sweden made an attempt to gradually introduce the Gregorian Calendar in 1700, by adopting a special Swedish calendar. However the change over didn't take place until 1753, where February 17 was followed by March 1.

The Gregorian calendar's year is divided into 12 months:

No.	Name	Length (days)
1	January	31
2	February	28 or 29
3	March	31
4	April	30
5	May	31
6	June	30
7	July	31
8	August	31
9	September	30
10	October	31
11	November	30
12	December	31

An average year is 365.2425 days = 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds.

A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.

A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.

(But some years may contain leap seconds.)

See also Common year starting on Sunday and Dominical letter.