the changed of the Gregorian Calendar in 1582?

Answer 1

The true length ofthe solar year is a trifle less than 365 and a quarter days - it is 365.242199 days, or 365 days, 5 hours, 48 minutes and 46 seconds, to be exact. Therefore the Julian calendar was too long by about 11 minutes; and after a number of centuries the error in question amounted to several days.
In the year 1582 another momentous calendar reform took place. Pope Gregory XIII determined to adjust the calendar to the seasons: The error caused by the excessive length of the Julian calendar now amounted to ten days. To set the year aright, there were ten days canceled from the Julian calendar, so that October 4,1852, was followed by October 15.
This, created confusion. And to avoid this confusion, dates prior to October 4, 1582, were given thereafter (and are still often given) as O.S. (old style) or N.S. (new style). If neither O.S. or N.S. is given after a date, the presumption is that it is N.S.
To avoid further error the leap year was changed. Only the centurial years (those ending with "00") that were divisible by 400 were to be leap years. Thus 1600 was a leap year, while 1700, 1800 and 1900 were not.
This Gregorian calendar is the one that we use today.

Answer 2

The Gregorian Calendar is the calendar that is in use today, with slight adjustments.
http://www.geocities.com/calendopaedia/gregory.htm

In 1582, it replaced the Julian Calendar (named after Julius Caesar), jumping ahead 11 days. October 4 suddenly became October 15.
England changed over in 1752 -- September 2, to be precise.
At that time, ths US was still part of England. Dates before then, like George Washington's birthday (1732), had to be adjusted by 11 days.