I saw a recent television commercial with a modern person at the Mt Vesuvius eruption asking a Pompeii resident the year. The resident answers with 79 AD. But I thought the zero point for the BC/AD system, the birthdate of Jesus, was reversed engineered centuries later. So what would the resident have said (modulo the conversation would have been in Latin(?) instead of English) as the year?
For such at mighty and well-organised empire, ancient Rome actually had a remarkably bad system for identifying a given year. Multiple methods were used, but the most widely used format seems to have been to name each year after the more senior of that year's two consuls. Another common method was to calculate the number of years since the traditional founding of Rome in 753 BC, but, perhaps because Roman numerals become a bit nightmarish once you get into the hundreds, this method doesn't seem to have been used as frequently as you might imagine. In the late empire, it became increasingly common to use regnal numbers, e.g. in the fifth year of Diocletian.
Returning to the whole BC:AD thing, in Luke 2.2, the author states that Jesus was born in the year of the 'first census that took place while Quirinius was governor of Syria'. The fact that the author of Luke's Gospel had to identify the year by referencing a provincial official testifies to the lack of a robust, uniform system of labelling years within the Roman Empire of the first century AD.