Gregorian Calendar

Gregorian Calendar was the son of Calendar.
Both the Gregorian Calendar and Julian Calendar are based on 12 months with a regular year of 365 days and leap years of 366 days. In the Julian Calendar every year which is divisable by 4 is a leap year, giving an average year length of 365.25 days.

The Gregorian Calendar modified the Julian Calendar sligihtly to correct much of the error it had in the length of the year, as it had been noted that the equinox and soltice had been drifting. The Gregorian Calendar dropped 3 leap years every 400 years (omitting the leap year every centery that isn't a multiple of 400) dropping the length of the year from 365.25 days to 265.2425 days, which is significanty closer to the real length of the average year. The current error is on the order of one day per 3000-7000 years and one proposal that has been made is we should drop 1 more day per 4000 years.

Table of the relationship between Julian Calendar and Gregorian Calendar

Julian Calendar	Gregorian Calendar	Difference
Until 1 Mar 100	Until 28 Feb 100	2 Days
2 Mar 100 - 29 Feb 200	1 Mar 100 - 28 Feb 200	1 Days
1 Mar 200 - 28 Feb 300	1 Mar 200 - 28 Feb 300	0 Days
29 Feb 300 - 27 Feb 500	1 Mar 300 - 28 Feb 500	1 Days
28 Feb 500 - 26 Feb 600	1 Mar 500 - 28 Feb 600	2 Days
27 Feb 600 - 25 Feb 700	1 Mar 600 - 28 Feb 700	3 Days
26 Feb 700 - 24 Feb 900	1 Mar 700 - 28 Feb 900	4 Days
25 Feb 900 - 23 Feb 1000	1 Mar 900 - 28 Feb 1000	5 Days
24 Feb 1000 - 22 Feb 1100	1 Mar 1000 - 28 Feb 1100	6 Days
23 Feb 1100 - 21 Feb 1300	1 Mar 1100 - 28 Feb 1300	7 Days
22 Feb 1300 - 20 Feb 1400	1 Mar 1300 - 28 Feb 1400	8 Days
21 Feb 1400 - 19 Feb 1500	1 Mar 1400 - 28 Feb 1500	9 Days
20 Feb 1500 - 18 Feb 1700	1 Mar 1500 - 28 Feb 1700	10 Days
19 Feb 1700 - 17 Feb 1800	1 Mar 1700 - 28 Feb 1800	11 Days
18 Feb 1800 - 16 Feb 1900	1 Mar 1800 - 28 Feb 1900	12 Days
17 Feb 1900 - 15 Feb 2100	1 Mar 1900 - 28 Feb 2100	13 Days.