Maybe at some point they switched to BC/AD, but what did they use before that? For example, what year, according to Roman timekeeping, was Julius Caesar killed?
hi! you may be interested in this section of the FAQ (link on sidebar):
The short answer is that the Romans referred to years by the names of the consuls for that year, or the first pair of consuls in years when there were more than one pair of appointees. So the year of Julius Caesar's death would be referred to as "in the consulship of Gaius Julius Caesar and Marcus Antonius", or in Latin "C. Iulio Caesare et M. Antonio consulibus". Lists of consuls -- the fasti consulares -- were kept to assist in referring to specific years.
Variants:
Some Roman historians, like Fabius Pictor, Varro, and Censorinus, used the Greek Olympiad count instead, which had the virtue of being the system used by the standard Greek chronology of history established by Eratosthenes. So Caesar's death could also be referred to as taking place in the 1st year of the 184th Olympiad oops, 4th year of the 183rd Olympiad, since the Greek year began in midsummer.
Starting in the Imperial period, it also became common (though not regulatory until very late antiquity) to refer to the nth year of an emperor's reign.
Also starting in the Imperial period, there are a handful of Roman historians who refer to the number of years since the supposed date of the founding of Rome, or n years "ab urbe condita", according to the count calculated by Varro during Augustus' reign.