Before the year 525, the years were counted from the foundation of Rome, according to the rules of the Julian calendar. At the behest of Pope John I, calculations were made to determine the year of Christ's birth as the beginning of the new calendar. Since the concept of zero was not yet known in Europe at that time, the calendar begins with the year 1 AD. The individual countries gradually adopted this dating system, and the last of the Western European countries to adopt it was Portugal in 1422.
