The Gregorian calendar is the solar dating system, based on a 365-day year with leap years added in most years divisible by four, that serves as the civil calendar of nearly every country in the world. It was promulgated by Pope Gregory XIII in the 1582 papal bull Inter gravissimas as a correction to the Julian calendar, whose slightly too-long year had caused calendar dates to drift out of alignment with the seasons and with the astronomical basis for calculating Easter. Adoption of the new calendar for civil purposes was not simultaneous; it proceeded unevenly across the world over more than three centuries, tracking religious and political lines as much as astronomical ones. The terms “Old Style” (O.S.) and “New Style” (N.S.) are conventionally used to disambiguate Julian and Gregorian dates during the long transition period.
The Gregorian calendar is today the de facto and de jure civil calendar of nearly all countries. Only Ethiopia, Nepal, Iran, and Afghanistan do not use it as their official civil calendar, relying instead on the Ethiopian calendar, the Vikram Samvat/Nepal Sambat, and the Solar Hijri calendar, respectively. Most Eastern Orthodox churches, along with some Eastern Catholic and Oriental Orthodox communities, continue to use the Julian calendar or the Revised Julian calendar for liturgical purposes-particularly for fixing the date of Easter-even in countries that use the Gregorian calendar civilly. This produces a recurring, well-documented divergence between Orthodox and Western dates for moveable feasts and, for churches still on the unmodified Julian calendar, for fixed feasts such as Christmas as well.
The reform itself addressed two related problems. First, the Julian year of 365.25 days is about 11 minutes longer than the mean tropical year, causing a drift of roughly three days every four centuries. By the late 16th century this drift had pushed the observed vernal equinox about ten days earlier than the calendar's traditional date. Second, this drift was steadily moving the date of Easter away from its intended relationship to the equinox, a concern dating back centuries within the Catholic Church. The reform addressed the first problem by changing the leap-year rule-century years are leap years only if divisible by 400, so 1700, 1800, and 1900 were not leap years while 1600 and 2000 were-and addressed the immediate drift by removing ten days from the calendar in October 1582. For Christoph Clavius's role in finalizing and defending the reform, and for the broader history of its drafting and adoption, see Gregorian Calendar History.
There is broad consensus among historians of science and astronomers that the Gregorian leap-year rule more closely approximates the length of the mean tropical year than the Julian rule did, and that the calendar reform achieved its stated technical aim of slowing the drift between the calendar year and the solar year. This is a narrow, technical consensus about calendrical accuracy; it does not extend to agreement on the legitimacy of papal authority over civil timekeeping, the wisdom of specific national adoption timelines, or the proper handling of dates during the transition period, all of which remain matters of viewpoint or historical debate. See gregorian-calendar-accuracy-consensus.