The other regions include the original 13 states and westward expansion thereof, which converted in 1752 with England. Louisiana Territory states converted with France (a Catholic country), in 1582.
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The difference is religion: the Gregorian calendar is named after Pope Gregory, and its adoption was hindered on account of objection to the Church, despite the technical improvement of the Gregorian calendar over the Julian, neatly pressaging DeSitter's objections to Debian on similar illogical religious grounds, despite technical advantages....