The idea of counting years has been around for as long as we have written records, but the idea of syncing up where everyone starts counting is relatively new. Today the international standard is to designate years based on a traditional reckoning of the year Jesus was born — the “A.D.” and "B.C." system.
"A.D." stands for anno domini, Latin for “in the year of the lord,” and refers specifically to the birth of Jesus Christ. "B.C." stands for "before Christ." In English, it is common for "A.D." to precede the year, so that the translation of "A.D. 2014" would read "in the year of our lord 2014." In recent years, an alternative form of B.C./A.D. has gained traction. Many publications use "C.E.," or "common era," and "B.C.E.," or "before common era." Before we talk about how and why the system was invented, let's get some historical context.
When is Easter?
In the early Middle Ages, the most important calculation, and thus one of the main motivations for the European study of mathematics, was the problem of when to celebrate Easter. The First Council of Nicaea, in A.D. 325, had decided that Easter would fall on the Sunday following the full moon that follows the spring equinox. Computus (Latin for computation) was the procedure for calculating this most important date, and the computations were set forth in documents known as Easter tables. It was on one such table that, in A.D. 525, a monk named Dionysius Exiguus of Scythia Minor introduced the A.D. system, counting the years since the birth of Christ.
Anno Diocletiani to Anno Domini
Dionysius devised his system to replace the Diocletian system, named after the 51st emperor of Rome, who ruled from A.D. 284 to A.D. 305. The first year in Dionysius' Easter table, “Anno Domini 532,” followed the year “Anno Diocletiani 247.” Dionysius made the change specifically to do away with the memory of this emperor who had been a ruthless persecutor of Christians.
Dionysius never said how he determined the date of Jesus' birth, but some authors theorize that he used current beliefs about cosmology, planetary conjunctions and the precession of equinoxes to calculate the date. Dionysius attempted to set A.D. 1 as the year of Jesus Christ’s birth, but was off in his estimation by a few years, which is why the best modern estimates place Christ’s birth at 4 B.C. [Related: Easter Science: 6 Facts About Jesus]
Adding in the years before Christ
The addition of the B.C. component happened two centuries after Dionysius, when the Venerable Bede of Northumbria published his "Ecclesiastical History of the English People" in 731. Up until this point, Dionysius’ system had been widely used. Bede’s work not only brought the A.D. system to the attention of other scholars, but also expanded the system to include years before A.D. 1. Prior years were numbered to count backward to indicate the number of years an event had occurred “before Christ” or “B.C.”
No Year Zero?
According to Charles Seife in his book "Zero: The Biography of a Dangerous Idea": “To Bede, also ignorant of the number zero, the year that came before 1 A.D. [sic] was 1 B.C. There was no year zero. After all, to Bede, zero didn’t exist.”
However, zero did exist; our modern conception of zero was first published in A.D. 628 by the Indian scholar Brahmagupta. The idea would not spread to medieval Christian Europe, however, until the 11th to 13th centuries.
Spread of the system
The B.C./A.D. system gained in popularity in the ninth century after Holy Roman Emperor Charlemagne adopted the system for dating acts of government throughout Europe.
By the 15th century, all of Western Europe had adopted the B.C./A.D. system. The system's inclusion was implicit in the 16th-century introduction of the Gregorian calendar, and it later would become an international standard in 1988 when the International Organization for Standardization released ISO 8601, which describes an internationally accepted way to represent dates and times.
Common and vulgar eras
The alternative form of “Before the Common Era” and “Common Era” dates back to 1715, where it is used in an astronomy book interchangeably with “Vulgar Era.” At the time, vulgar meant “ordinary,” rather than “crude.” The term “Vulgar Era” is even older, first appearing in a 1615 book by Johannes Kepler.
Rationales for the transition from A.D. to C.E. include (1) showing sensitivity to those who use the same year number as that which originated with Christians, but who are not themselves Christian, and (2) the label “Anno Domini” being arguably inaccurate, since scholars generally believe that Christ was born some years before A.D. 1 and that the historical evidence is too sketchy to allow for definitive dating.