Are there any historical records marking the official switch to the Christian system of yearkeeping? Was the first day of the first year self aware of its novelty, or was this system set in place retroactively?
I can recommend this thread with answers from u/DanKensington (linking to the FAQ) and u/MichaelJTaylorPhD. Though the AD calendar was invented in Byzantium in the 500s, it was still, as u/sunagainstgold has noted in an answer in the FAQ, only one of several systems for most of the Middle Ages.