The Christian Calendar

In 1267, the medieval scientist Roger Bacon stated the times of full moons as a number of hours, minutes, seconds, thirds, and fourths (horae, minuta, secunda, tertia, and quarta) after noon on specified calendar dates. Although a third for 1/60 of a second remains in some languages, the modern second is further divided decimally.

Rival calendar eras to Anno Domini remained in use in Christian Europe. In Spain, the "Era of the Caesars" was dated from Octavian's conquest of Iberia in 39 BC. It was adopted by the Visigoths and remained in use in Catalonia until 1180, Castille until 1382 and Portugal until 1415.

For chronological purposes, the flaw of the Annon Domini system was that dates have to be reckoned backwards or forwards according as they are BC or AD. According to the Catholic Encyclopedia, "in an ideally perfect system all events would be reckoned in one sequence. The difficulty was to find a starting point whence to reckon, for the beginnings of history in which this should naturally be placed are those of which chronologically we know least."

For both Christians and Jews, the prime historical date was the Year of Creation, or Annus Mundi. The Byzantine Church fixed the date of Creation at 5509 BC. This remained the basis of the ecclesiastical calendar in the Greek and Russian Orthodox world until modern times. The Coptic Church fixed on 5500 BC. Later, the Church of England, under Archbishop Ussher in 1650, would pick 4004 BC. Jewish scholars preferred 3761 BC as the date of creation, which forms the basis of the modern Jewish calendar. However, "any attempt thus to determine the age of the world has been long since abandoned."

Read how the ancient calendars started.