Gregorian calendar

Why it was introduced

The Julian calendar, devised earlier, assumed a year length of 365.25 days by adding an extra day every four years. Because the true tropical year — the average time for the Earth to complete one orbit of the Earth's path around the Sun from one equinox to the next — is slightly shorter than 365.25 days, the Julian year gained a few minutes each year relative to the seasons. Over centuries this discrepancy shifted the calendar date of the spring equinox and thus the date used to compute movable feasts such as Easter.

Reform and rules

Pope Gregory XIII promulgated a reform that omitted ten days to realign the calendar with the seasons and introduced a refined leap-year rule to reduce future drift. The skip restored the equinox to its traditional date in church practice and set a simple rule for most civil calculations.

• Most years have 365 days.
• Ordinarily, a leap year occurs every 4 years, adding one extra day (February 29).
• To correct for the Julian overcount, years that are divisible by 100 are not leap years, unless they are also divisible by 400. This preserves long-term alignment of the calendar and the seasons.

Under these rules, years like 1700, 1800 and 1900 were common rather than leap years, while 1600 and 2000 were leap years because they are divisible by 400.

Adoption and historical effects

The initial adjustment took place in 1582: several Catholic countries advanced their dates by skipping ten days in October so that Thursday the 4th was followed by Friday the 15th. Adoption in other countries occurred more gradually. Protestant and Orthodox regions accepted the reform at different times, with some countries changing calendars only in the 18th or 20th centuries. The staggered adoption produced occasional confusion in international affairs, legal records and historical dating until the Gregorian calendar became globally predominant for civil purposes.

Structure, uses and distinctions

The Gregorian calendar preserves the familiar 12-month structure with months of 28–31 days. It is primarily a solar calendar designed to track the cycle of seasons and has been used for civil administration, commerce, timekeeping and the scheduling of religious observances. While it is the default civil calendar for most countries, other calendar systems continue to be used for religious or cultural purposes alongside it.

Notable facts

Because the civil reform was aimed at synchronizing the liturgical year with astronomical events, the Gregorian calendar is often discussed in contexts that reference the older Julian calendar, the notion of a global civil calendar, or the concept of common years. Its leap-year design offers a close approximation to the tropical year: the small residual discrepancy is only a few seconds per year on average, which accumulates much more slowly than with the Julian system.

Because of its balance of simplicity and improved accuracy, the Gregorian calendar remains the most widely used civil calendar in the contemporary world and provides the basis for international timekeeping, business calendars and most legal systems.