The calendar introduced by Julius Caesar and used in Russia at the time of the Russian Revolution while the rest of the world was 13 days ahead on the Gregorian calendar. This is why the 1917 revolutions are known as the February and October in Russia, and the March and November in the rest of the world. The Gregorian calendar was introduced by Pope Gregory XIII in 1582 as a correction to the Julian and is used throughout the world today.
