I'm not sure this is entirely correct, it implies a level of standardisation that I don't think was the case. Not everyone agreed when years were considered to

I'm not sure this is entirely correct, it implies a level of standardisation that I don't think was the case. Not everyone agreed when years were considered to have started or used the same numbering systems for a very long time, even after the Julian calendar was widely adopted. You can see how this played out in England, a relative latecomer to the increasing year start consensus. Up until the Calendar (New Style) Act of 1750, the year was considered to start on the 25th of March. This means that historical documents in England containing dates in January or February have what we would consider to be the 'wrong' year number on them. Pretty inconvenient, but at least the roman numbered months would have made some sense to a person living in England before 1750.