The Gregorian Calendar is introduced by Pope Gregory XIII.

The **Gregorian calendar** is the calendar system used by most of the world today. It was introduced in **1582 by Pope Gregory XIII** as a reform of the earlier Julian calendar, which had been established under Julius Caesar in 45 BC. The Julian calendar treated a year as exactly 365.25 days, using a leap year every four years. However, the actual solar year is slightly shorter—about 365 days, 5 hours, 49 minutes. Although the difference was only around 11 minutes each year, over many centuries it caused the calendar to drift away from the astronomical seasons. By the 16th century, this discrepancy had accumulated to roughly ten days and was affecting the calculation of important Christian observances, particularly Easter.

The Gregorian reform corrected the problem in two main ways. First, **ten calendar days were removed**: in countries that immediately adopted the reform, Thursday, October 4, 1582, was followed by Friday, October 15. Second, the rules governing leap years were changed to make the average calendar year more closely match the solar year. Under the Gregorian system, years divisible by four are normally leap years, but century years must also be divisible by 400. Thus, **1600 and 2000 were leap years, while 1700, 1800 and 1900 were not**. This produces an average year of 365.2425 days, making the Gregorian calendar far more accurate than its Julian predecessor.

Adoption of the Gregorian calendar was gradual because the reform initially came from the Roman Catholic Church. Catholic countries such as Spain, Portugal, Italy and Poland adopted it in 1582, while many Protestant and Orthodox countries resisted the change for decades or even centuries. **Great Britain and its colonies adopted the Gregorian calendar in 1752**, when September 2 was followed by September 14, eliminating eleven days. Russia did not adopt it for civil purposes until 1918, following the Russian Revolution, while Greece changed in 1923. Today, the Gregorian calendar serves as the principal international civil calendar and provides a common system for dating business, government, travel and everyday life across much of the world.