We refer to the calendar system that is in most widespread international use around the world today as the Common Calendar. The Common Calendar consists of two parts: it uses the rules of the Julian Calendar for dates up to 4 October 1582, and the rules of the Gregorian Calendar for dates starting with 15 October 1582. In the Common Calendar, 15 October 1582 was the day immediately following 4 October 1582. The era of this calendar is referred to as the Common Era (C.E.).