Most countries use the Gregorian calendar for civil dates and official business.

The longer version

The civil calendar in use nearly everywhere is the one introduced by Pope Gregory in 1582, refined from the older Julian scheme. Its rules for leap years keep the average year close enough to the real orbital period that seasons stay put for many centuries. It was designed so the date of Easter would remain correct.

Adoption was gradual and political. Catholic states switched first, Protestant and Orthodox regions resisted for decades or centuries, and some countries changed only in the twentieth century. Religious bodies often kept older calendars for feast days, which is why certain holidays fall on different dates in different places. Today it functions as a neutrally shared standard for trade and law.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.