AD

The abbreviation AD (or A.D.) stands for the Latin phrase Anno Domini, which means "the year of our Lord (Jesus Christ)". It's used to designate years in the Christian calendar, which is used both in Mythic Europe and in the real world. (In modern, real-world usage, the abbreviation "CE" for "Common Era" is sometimes used).

ArM5 writes all dates as A.D. This makes it easy to correlate fictional events in Mythic Europe with Real History. Earlier editions of Ars Magica recorded most dates in terms of the Age of Aries.