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.

• Reform-as-necessary-correction: holds that the calendar drift was a genuine technical problem requiring correction, and that the Gregorian solution was a sound application of astronomical knowledge to a practical civic and religious need. See gregorian-calendar-necessity-viewpoint.
• Reform-as-papal-overreach: a view common among early modern Protestants, holding that the calendar change was illegitimate not on astronomical grounds but because it originated from papal authority, which Protestant polities did not recognize as binding on civil or ecclesiastical matters outside the Catholic Church. See gregorian-calendar-papal-authority-skeptical-viewpoint.
• Gradualist-pragmatist view: holds that despite religious objections, the practical advantages of calendar uniformity with trading partners and neighboring states eventually outweighed confessional concerns, explaining the staggered but eventual near-universal adoption. See gregorian-calendar-pragmatist-viewpoint.
• Orthodox liturgical-continuity view: holds that civil adoption of the Gregorian calendar is acceptable or even necessary for practical coordination with the rest of the world, but that the Julian calendar should be retained for liturgical purposes to preserve traditional dating of feasts and continuity with historical practice. See gregorian-calendar-orthodox-liturgical-viewpoint.
• Calendar-reform-skeptic view: a minority position, held by some scientists and scholars contemporaneous with the reform (such as Joseph Justus Scaliger and François Viète) and occasionally echoed since, questioning whether the specific reckonings adopted by the Clavius commission were the most accurate available at the time, or whether further reform (such as a fully astronomical rather than tabular Easter calculation) would have been preferable. See gregorian-calendar-accuracy-skeptical-viewpoint.

• Dispute between Christoph Clavius and François Viète over whether the Gregorian calculations were astronomically sound, conducted in published exchanges in the years following the reform. See gregorian-calendar-viete-clavius-controversy.
• The “calendar riots” said to have accompanied Britain's 1752 switch, including disputed claims that crowds demanded the return of the “stolen” eleven days. See gregorian-calendar-1752-britain-riots-controversy.
• The 20th-century schisms within Eastern Orthodoxy over adoption of the Revised Julian calendar for fixed feasts, which produced lasting splits between “New Calendarist” and “Old Calendarist” factions in several national churches. See gregorian-calendar-orthodox-calendar-schism-controversy.

• History: Gregorian Calendar History
• Consensus: gregorian-calendar-accuracy-consensus
• Controversy: gregorian-calendar-viete-clavius-controversy
• Related Main Topic: julian-calendar
• Related Main Topic: easter-dating-controversy-history
• Related Main Topic: Heliocentrism - Catholic Church's Historical Viewpoint

1. Pope Gregory XIII, Inter gravissimas (Rome, 1582).
2. Christoph Clavius, Romani calendarii a Gregorio XIII. P.M. restituti explicatio (Rome, 1603).
3. J. L. Heilbron, The Sun in the Church: Cathedrals as Solar Observatories (Cambridge, MA: Harvard University Press, 1999), 24-46, 144-175.
4. Reijer Hooykaas, “The Reception of Copernicanism in England and the Netherlands,” in The Copernican Achievement, ed. Robert S. Westman (Berkeley: University of California Press, 1975).
5. “Adoption of the Gregorian Calendar,” Encyclopaedia Britannica, accessed June 26, 2026, https://www.britannica.com/topic/Gregorian-calendar.
6. “List of Adoption Dates of the Gregorian Calendar by Country,” Wikipedia, accessed June 26, 2026, https://en.wikipedia.org/wiki/List_of_adoption_dates_of_the_Gregorian_calendar_by_country.
7. “Inter gravissimas,” Wikipedia, last modified January 24, 2026, https://en.wikipedia.org/wiki/Inter_gravissimas.
8. “Revised Julian Calendar,” Wikipedia, last modified April 28, 2026, https://en.wikipedia.org/wiki/Revised_Julian_calendar.
9. Reuven Bien, “Viète's Controversy with Clavius over the Truly Gregorian Calendar,” Archive for History of Exact Sciences 61, no. 1 (2007): 39-66.