When did the Gregorian calendar become standard in its country?

Italy, Spain, Portugal and Poland were the first four countries to switch from the Julian calendar, the calendar reformation commissioned by Julius Caesar in 46 BCE, to the Gregorian calendar, the calendar reformation commissioned by Pope Gregory XIII in 1582. They started using the Gregorian calendar on the 15th of October 1582.

The last country to switch from the Julian calendar to the Gregorian calendar was Turkey, more than 344 years later.