Gregorian calendar

The Gregorian calendar went into effect in October 1582 after a papal decree of Pope Gregory XIII. It fixed the Julian calendar's drift by dropping days and by skipping leap years in century years not divisible by 400.

Catholic countries took it up first. Britain and its colonies, including those that became the United States, switched in September 1752, and other countries as late as the twentieth century. When comparing dates across borders before 1752, check which calendar each record used.