It’s actually worse than that. It wasn’t always whole coubtries who decided to adopt (or not) but cities and sometimes people within cities (i.e. the protestants in the city would be lagging, or maybe I’m misremembering and this was about people who where abroad)

In any case, for awhile, the date you picked depended on who you were writing to. And then also the relative standing. If he was of much lower standing you might force your own calendar on them.

Also, I think with the previous calendar it was always a bit debatable what year december belonged to. I can’t quite remember the details.