In 1712, Sweden had a 30th of February.
From 1700 Sweden tried to ease from Julian to Gregorian by skipping every leap day for 40 years. Then the Great Northern War came, and in 1704 and 1708 the skipping was forgotten. Sweden now matched neither calendar. In 1712 it returned to Julian by adding a second leap day: 30 February. Gregorian came only in 1753.
Useful: A half reform is often worse than none; if you switch, do it in one day.
Source: en.wikipedia.org