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The History of the Gregorian Calendar

The Gregorian calendar is the most widely used calendar system in the world today. In this system, a standard year has 365 days, with an extra day—February 29—added during a leap year. April, June, September, and November each have 30 days, while the remaining months have 31 days, except for February, which has 28 days in a standard year and 29 in a leap year.

This calendar is a refined version of the Julian calendar, which itself evolved from the ancient Roman calendar. The earliest Roman calendar was likely a lunar system based on the phases of the moon. Initially, the Romans used a 10-month calendar consisting of 304 days, with the remaining days of winter left uncounted. This imprecise system caused seasonal drift, prompting the need for more accurate calendar reforms.

Later, the Roman Republic adopted a calendar influenced by the Greek system, which recognized that a lunar month was approximately 29.5 days and that 12.5 such months made up a solar year. To reconcile this, intercalary months—January and February—were periodically added, especially every fourth year, to realign the calendar with the solar cycle. However, even this system proved inconsistent.

In 46 BC, Julius Caesar introduced a more substantial reform. He eliminated the need to base the calendar on moon sightings and added 10 days to the year, resulting in a 365-day calendar. He also established a leap year every four years by adding an extra day, thus better aligning the calendar with the solar year. This reform created what became known as the Julian calendar.

Yet, the Julian calendar still had a small error: it miscalculated the solar year by about 11 minutes annually. Over time, this discrepancy caused the calendar to drift out of sync with the equinoxes and solstices. By 1582, this error had accumulated to a ten-day shift. To correct this, Pope Gregory XIII implemented the Gregorian calendar. He advanced the calendar by skipping ten days—so the day after October 4, 1582, became October 15—and revised the leap year rule: century years would only be leap years if divisible by 400. This correction significantly reduced the error rate from one day every 128 years to one day every 3,030 years.

Adoption of the Gregorian calendar was gradual, with different countries making the switch over several centuries. Despite occasional proposals for further reform, the Gregorian calendar remains the dominant system for civil use around the globe.