The Gregorian calendar, which is now used worldwide, began as the Roman Catholic calendar. It reckons years as A.D. (anno Domini, "in the year of the Lord") or B.C. ("before Christ"). Many non-Christian religions retain their own calendars for religious purposes. For example, the Hebrews reckon years from the Creation, Zoroastrians reckon from the birth of Zoroaster, and Muslims reckon from the flight of Mohammed from Mecca (similar to the Nephites reckoning from the flight of Lehi from Jerusalem).
How, then, has this Christian calendar become almost universally accepted as the legal calendar in a predominantly non-Christian world? The answer is apparently twofold: first, the calendar spread around the world with Christian colonization; second, as international trade increased, it became convenient for everyone to use the same calendar.
In A.D. 325 the Council of Nicaea adopted the Roman calendar as the official calendar of the Roman Catholic Church. Nearly identical to our calendar today, this calendar is called the Julian calendar in honor of Julius Caesar, who introduced it. It was not until two centuries later that our current system of reckoning years from the birth of Christ was devised. At that time a Catholic monk, Dionysius Exiguus, calculated the year of Christ's birth from the available records and proposed that the Christian Era begin in the year now called A.D. 1.
This method of reckoning time from Christ was not widely used until Charlemagne made it official for the Holy Roman Empire in the ninth century A.D. By the twelfth century, England had also begun using the system, which spread around the world with the European colonization that followed.
In 1582 Pope Gregory XIII made minor changes to the Julian (or "Old Style") calendar to make it astronomically more accurate so that Easter would be celebrated at the right time. Thus the calendar became known as the Gregorian (or "New Style") calendar, and the A.D. reckoning became firmly established as the calendar was adopted by Roman Catholic countries such as Spain and France.
Later the Gregorian calendar became less associated with Catholicism and gained popularity because it was accurate and convenient for international trade. It was adopted by several Protestant countries such as those in Scandinavia in 1700, Great Britain in 1752, and countries arising from Great Britain's worldwide colonies, such as America in 1752 and India in 1757. China adopted the same calendar in 1911, Russia in 1918, and some Eastern Orthodox countries as late as 1940.
Today virtually the entire world uses the Gregorian calendar for commercial purposes. In order to remove the Christian implications from the calendar, the designations C.E. ("of the common era") and B.C.E. ("before the common era") are sometimes used to replace A.D. and B.C.
It is interesting to note that the Nephites were probably the first nation to reckon years from the birth of Jesus Christ, doing so as early as nine years after the sign of his birth. (See 3 Ne. 2:8.) Thus both world hemispheres independently began reckoning time from Christ's birth.
There have been various attempts to replace the Gregorian calendar so as to not use Christ's birth as the beginning point. In 1793 France began counting years from the French Revolution the previous year, abandoning the seven-day week and dividing each thirty-day month into three periods of ten days each. This system was discontinued on 1 January 1806 after Napoleon Bonaparte, in order to be recognized by the Pope as the emperor of France, agreed to return to the Gregorian calendar. A more recent example is Sri Lanka. When that country gained independence in 1966, it opted to revert to the Buddhist calendar, but in 1971 it returned to the Gregorian calendar because of difficulties in international commerce.
In sum, the spread of Christian colonization established the calendar we use today, and international commerce continues to motivate its use.