In ancient Rome years were counted by the names of the two consuls who ruled that particular year. If the consul(s) ruled longer than one year, then it would be the first, second, etc, year of his/their rules. Thus, Caesar was assassinated in the fifth year of his own consulship and the first year of that of Marcus Antonius.
Marcus Terentius Varro (116 BCE-27? BCE) attempted to introduce a system of counting the years that would be independent of consuls' names, and would, therefore, make it easier to determine what happened first, what second, etc. He determined that the city of Rome was funded on April 21, 753 BCE and proposed that calendar should be based on that date. That would transfer Caesar's assassination from the 5th/1st year to 710 ab urbe condita. Despite Varro's noble effort Romans continued to use the consular calendar until 537 CE when Emperor Justinian I decreed that it should be the year of the emperor's rule that would be the basis of the calendar.
The relativism of Roman calendar makes it often hard to determine when exactly certain events occurred.
Titus Livius (59? BCE-17 CE) took Varro's idea seriously around 29 BCE when he started working on his 142-volume history of Rome, which he titled Ab Urbe Condita. Not all of his volumes have survived, we only know about most of them from indirect sources. However, volumes I-X and XXI-XLV (that's 1-10 and 21-45) have been preserved to this day.