Who invented the B.C./A.D. system, and when?  

Dionysus Exiguus, in the Sixth Century (A.D.) 

In the sixth century (A.D.), this monk and scholar was asked by the Pope to work out when Jesus Christ was born, so that a calendar or dating system could be introduced which placed events according to the number of years they occurred either before or after the birth of Christ. This system came to be used in Christendom, instead of using the founding of Rome (753 B.C.) as the reference point, or "year zero."

In Russia, until 1700 A.D., the Orthodox Church counted from the supposed creation of the world (then thought to be about 4000 B.C). In non-Christian countries, other calendars were (and are) used, again, often based on religious events. Calendars did not have to be based on religious events. During the French Revolution, it was decided to bring in the Revolutionary calendar, with "year zero", dating from the start of the revolution in 1789. A few years later the French returned to using B.C. and A.D.

Dionysus made 2 mistakes; firstly, he got the date of Christ’s birth wrong by 4 years- it is now thought that Christ was born no later than 4 B.C., and secondly, he didn’t include a year for the first year of Christ’s life- there should really have been a "year zero", between 1 B.C. and 1 A.D.

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