>Of course, you have to then ask "in which country?" As you point out, some countries didn't make the switch until much later.
... adding to the confusion, I know.
According to a book I read about calenders ("Der Kalender im Wandel der Zeiten"), the reason many countries didn't adopt the Gregorian Calendar initially stemmed from a mistrust against the Catholic Church.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)