Which calendar was introduced by Pope Gregory XIII in 1582 and is now widely used for New Year's Day?

The Gregorian calendar was introduced by Pope Gregory XIII in 1582 and is now widely used for New Year's Day.

The reform corrected a drift in the Julian calendar, whose estimate of the solar year was slightly too long. Over centuries, that small difference shifted calendar dates relative to the seasons. The Gregorian reform removed 10 calendar dates in the first countries that adopted it and adjusted the leap-year rule.

Under the Gregorian rule, century years are not leap years unless divisible by 400. Thus 1600 and 2000 were leap years, while 1700, 1800, and 1900 were not. This makes the calendar more closely track the tropical year than the Julian system.

Adoption was not worldwide or simultaneous. Catholic countries generally adopted it first, while some Protestant and Orthodox countries changed later. The Russian Empire adopted it after the 1917 Revolution, and Greece adopted it in 1923 for civil use. Today, January 1 is the common civil New Year's Day in most countries.