... And then there was Sweden.

Instead of flipping the switch in the 1500s... or 1700s... they instead did a slow roll out until they finally flipped the switch and did a big leap.

https://www.americanscientist.org/article/date-distinctions

    In “The Future of Time,” the authors state that “different countries and religions adopted the reforms [the Gregorian calendar] at different times and in different ways. In 1712 Sweden added a February 30 to its national calendar.” There was a February 30 in Sweden in 1712, but the context in which it is presented in the article is misleading. In 1700 the Swedes decided to switch from the Julian to the Gregorian calendar by eliminating leap-year days for 40 years. So 1700, which is a Julian leap year but not a Gregorian leap year, did not have a leap day. In both 1704 and 1708, through bureaucratic error, there were leap days. In 1712, the situation was resolved by switching back to the Julian calendar; this meant that the missing leap day that would have occurred in 1700 had to be added, and was, as February 30. It wasn’t until 1753 that the Swedes finally adopted the Gregorian calendar by making the date March 1 follow February 17.