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On February 24, 1582, Pope Gregory XIII established the Gregorian calendar via a Pappel Bull, the primary calendar used today and underlying international commerce.
The new calendar went into effect on October 4, 1582. In 1752 it became the official calendar throughout most of Europe and the empire colonies. Russia, a holdout, started using it in the 20th century.
The Julian calendar, which the world was using, was created by Julius Cesar. It had a problem, though. It wasn't accurate. Every year it was off by 11.5 minutes. Over centuries, 11 minutes per year adds up. When Pope Gregory established the new calendar, Cesar’s was ten days off.
The Julian calendar added an extra day every four years to account for the extra quarter day it takes the sun to rotate around the earth. We call this a "leap year." This idea may have come from the Hebrew calendar, which adds a thirteenth month, Adar II, to account for the difference between the moon and sun rotations in its leap years.
The Gregorian calendar addressed the missing 11.5 minutes and split time into two eras: before the birth of Jesus Christ (BC) and after the nativity, or anno Domini (AD), "the [first] year of our Lord" in Latin. Of course, non-Christian religions and secularists didn't like this, so they changed it to "Before the Common Era" (BCE) and "Common Era" (CE). No matter what you call it, the demarcation point is still the same: the birth of Jesus Christ.
Orthodox Christians still use the Julian calendar (off by 11 days when the Gregorian calendar was adopted globally in 1752) as their religious calendar, which is why you'll find two different celebrations of Easter and Christmas each year. Other religions use their calendars, most of which follow the movements of the sun, moon, and-or stars. These calendars are shorter than the Gregorian, now the international standard.
The oldest calendar in the world is the Byzantine calendar, which began marking time over 7,500 years ago.
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