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. 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. Dionysius devised his system to replace the Diocletian system, named after the 51st emperor of Rome, who ruled from A. E.," or "before common era." Before we talk about how and why the system was invented, let's get some historical context. 325, had decided that Easter would fall on the Sunday following the full moon that follows the spring equinox. 525, a monk named Dionysius Exiguus of Scythia Minor introduced the A. system, counting the years since the birth of Christ.
He therefore proposed that the Christian era begins at the start of the following year. Such post facto chronologies are of little use for previous history.
system gained in popularity in the ninth century after Holy Roman Emperor Charlemagne adopted the system for dating acts of government throughout Europe. 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.
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.
But this does not make the new chronology any less useful for dating subsequent events, once the chronology is widely used (a process which takes some time to achieve) you have a firm framework.
Although the dating system was devised in 525, it wasn’t widely used until after 800,and even after that, other systems were still widely used throughout Europe.