The most commonly used calendar system in the world today. It is based on the older Julian calendar of the late Roman Empire but more accurately corresponds to the solar year of 365. 2422 days. The Julian calendar year is 365. 25 days. This is inaccurate by approximately 11 minutes a year. The result is that the Julian calendar is out of sync by one day every 131 years. The Gregorian calendar fixed this problem by changing the rules for leap years. In the Julian system, a day is added to the end of February every 4 years. In the Gregorian system a year is a leap year if it is divisible by 4 unless it is divisible by 100 but not by 400. This eliminates leap years periodically in order to keep the calendar more synchronized with the solar year. The Gregorian calendar was named after Pope Gregory XIII who officially approved it in 1582 A. D. England and its colonies did not adopt it until 1752.