These terms are chiefly found in modern Latin texts. English
speakers are unlikely to recognize them. Neither the Chicago
Manual of Style (14th ed.), the American Heritage Dictionary
(3rd ed.), nor P. Kenneth Seidelmann's Explanatory Supplement to
the Astronomical Almanac (1992, University Science Books)
mention AC, ACN, or Ante Christum Natum. These terms were not
used in medieval and Renaissance Latin texts. Bede the
Venerable, who was the first writer to identify a year as before
Christ, used the Latin ante incarnationis dominicae tempus
(before the time of the Incarnation of the Lord) in his Historia
ecclesiastica gentis Anglorum (I.2) in 731. Most comparable
early Latin terms referred to Christ's Incarnation or
conception, not his birth nine months later.