[LeftHandPath]

If the books are set in 55 BC, how would the characters know it was 55 BC?

[_9ptr]

Well, Caesar was born in 100 BC and they knew how old he was. Simple calculation, really.

[bdcravens]

From the future perspective, yes. From the perspective of characters at the time, they wouldn't use that nomenclature, since it didn't exist yet. They would rather use the numbering system of their time (in this case, years since the establishment of Rome)

https://en.wikipedia.org/wiki/Ab_urbe_condita

[dfxm12]

Likely for the same reason they speak modern French.