Greece becomes the last European country to adopt the Gregorian calendar.

The Gregorian calendar is the calendar system most of the world uses today to organize dates and time. It was introduced in 1582 by Pope Gregory XIII as a reform of the older Julian calendar, which had slowly drifted out of alignment with the solar year. That drift meant important dates, especially religious ones like Easter, were gradually shifting away from their intended seasons. The Gregorian calendar fixed this by more accurately matching the Earth’s orbit around the Sun.

The key improvement of the Gregorian calendar lies in how it handles leap years. A normal year has 365 days, but because a solar year is about 365.2422 days long, extra time needs to be accounted for. The calendar adds a leap day every four years, but with an important exception: years divisible by 100 are not leap years unless they are also divisible by 400. This rule makes the average calendar year much closer to the true solar year, reducing long-term drift to a tiny amount.

Although the calendar was created for religious reasons, its accuracy made it useful far beyond that context. Adoption was gradual, with Catholic countries switching first and others following over the next few centuries. Today, the Gregorian calendar is the international standard for civil use, supporting everything from global business and science to everyday planning. Its widespread acceptance shows how a carefully designed system can shape how societies coordinate time on a global scale.