Dates - The Gregorian Calendar

The Gregorian Calendar, also known as the Western or Christian Calendar, is the most widely used calendar in the world today. Its predecessor, the Julian Calendar, was replaced because it did not properly reflect the actual time it takes the Earth to circle once around the Sun, known as a tropical year.    

The reason the Julian Calendar had to be replaced was the formula it used to calculate leap years. The Julian formula produced a leap year every four years, which is too many. The Gregorian Calendar uses a much more accurate rule for calculating leap years.   

The papal bull issued by Pope Gregory XIII in 1582, decreed that 10 days be dropped when switching to the Gregorian Calendar. However, the later the switch occurred, the more days had to be omitted.  

This created short months with only 18 days and odd dates like February 30 during the year of the change over.   

Switch Took More Than 300 Years    

The Catholic countries of Italy, Poland, Portugal, and Spain immediately observed the calendar change, but for almost two hundred years Protestant countries refused to change to the new calendar because it had reformed by a Catholic Pope. The Greek Orthodox countries didn't make the change until the start of the 20th century.  

The Gregorian Calendar was first introduced in 1582 in some European countries. However, many countries used the Julian Calendar much longer. Turkey was the last country to officially switch to the new system on January 1, 1927.   

Other more recent converts were Russia, Latvia and Estonia (1918) and Bulgaria (1916) and Lithuania (1915) 

For more information:- 

http://www.tondering.dk/claus/cal/gregorian.php