Gregorian Calendar

The Gregorian calendar is a solar calendar, tracking the year of the seasons and taking no account of the moon in its months. It reckons 365 days in an ordinary year and 366 in a leap year, adding a day every fourth year but omitting the leap day in century years not divisible by four hundred, a refinement that keeps it very closely aligned with the true solar year. Introduced by Pope Gregory XIII in 1582 to correct a drift in the older Julian calendar, chiefly to fix the date of Easter, it was adopted gradually across the world and is now the international civil standard. Its era, Anno Domini or the Common Era, counts from a traditional date for the birth of Jesus. These are the features of the Gregorian calendar, the framework against which the other calendars on this site are commonly converted and compared.

Adoption was neither immediate nor uniform. Catholic states, Spain, Portugal, the Italian states and the Polish-Lithuanian Commonwealth, switched in October 1582 as Gregory XIII decreed, skipping ten days so that 4 October was directly followed by 15 October. Protestant and Orthodox states resisted for generations on confessional grounds: the German Protestant states did not convert until 1700, Britain and its colonies, including what became the United States, until 1752, skipping eleven days by then, Japan adopted it in 1873, and Russia only after the 1917 revolution, in February 1918, skipping thirteen days, which is why the Bolshevik "October Revolution" of 1917 is commemorated in November. The leap rule itself, a year divisible by four is a leap year unless divisible by 100, in which case it is not, unless it is also divisible by 400, in which case it is, yields an average year of 365.2425 days, accurate to within about 26 seconds of the true solar year.

Every calendar in the world solves the same arithmetic problem, and there are only three solutions. A lunation is about 29.53 days and a tropical year about 365.2422 days, and the second is not a whole multiple of the first. A calendar can therefore follow the Moon and let its year drift against the sun, follow the sun and let its months lose contact with the Moon, or follow both and insert an extra month at intervals to keep them together.

The Gregorian calendar takes the second path and abandons the Moon entirely. Its months are conventional divisions of a solar year and have no relation to any lunation, which is why the word month and the word moon share a root that the calendar no longer honours.

It is a correction rather than a new system. The Julian calendar it replaced assumed a year of exactly 365.25 days, about eleven minutes too long, and the accumulated error had moved the equinox by ten days by the sixteenth century. The reform promulgated by Pope Gregory XIII in 1582 dropped those ten days and changed the leap rule so that century years are leap years only when divisible by 400, giving a mean year of 365.2425 days.

Adoption was slow and followed confessional lines, so European dates from the sixteenth to the twentieth century may be given in either style and are sometimes marked Old Style or New Style to say which. The calendar carries one lunar remnant: the date of Easter is still computed from a lunar cycle, and the computation of it was a principal motive for the reform.