Adoption of the Gregorian calendar
The adoption of the Gregorian Calendar has taken place in the history of most cultures and societies around the world, marking a change from one of various traditional (or "old style") dating systems to the contemporary (or "new style") system – the Gregorian Calendar – which is widely used around the world today. Some polities adopted the new calendar in 1582, others not before the early twentieth century, and others at various dates between.
Source: Wikipedia — Adoption of the Gregorian calendar (CC BY-SA 4.0)