There wasn’t any real renumbering to be done when the BC/AD system was introduced. For some mysterious reason, the idea of simply numbering years in order seems to have been a very difficult one for humans to accept. Although the Romans had the A.U.C. dates, they didn’t normally use them – they were about as common as the “In the nth year of the Independence of the United States” dating you occasionally see on legal documents. Romans normally dated things, “The year when so-and-so and so-and-so held the office of Consul” (even after the Emperors held all the power, Consuls continued to be elected on paper), and dark-age Europe normally dated things, “The nth year of King so-and-so’s reign,” or worse.
In the late BC period, a serially-numbered calendar based on the life of Alexander the Great was in use for a while, but it seems to have vanished under Roman influence. Other systems were also occasionally attempted, usually by some Greek scholar or other, including one that was picked up by the Jews, and continues in Jewish use to this day. But in general, the BC/AD system was the first serially-numbered one to be widely used.
Even after BC/AD was determined on, the question remained of just when the year began. Until 1752, in England, the year ended on December 31, but began on March 25, leaving everyone uncertain as to just what to do with the days between. It still drives historians crazy when working with original documents.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams