I imagine that the use of "B.C." and "A.D." didn't start in what we would call the year 1, so what year would contemporary Romans say they were living in? Did it change between the republic and the empire? Was there even a concept of numbered years as we know them today?
Romans in the time of the Republic marked their years by naming the consuls of those years. For example, the year we now call "1 AD" was known as "the year of the consulship of Gaius Iulius Caesar^# and Lucius Aemilius Paullus". They kept very detailed records of their consuls: the "fasti consulares".
Later, when they had had Emperors for a while, they reverted to the more usual regnal year system, where years were identified as "In the seventh year of the reign of Emperor ...".
A similar question came up only yesterday: "How did people measure, mark, or denote years before the year "0" (B.C., A.D.), as they could not have predicted the "birth of Christ" to denote B.C. years?"
You may also be interested in the 'Other dating systems' section of the Popular Questions pages, as found in the sidebar.
^# ^(This Gaius Julius Caesar was the grandson of Augustus/Octavian; the offspring of Augustus's daughter and of Augustus's life-long friend Marcus Agrippa.)
