Not to be confused with the A.D. system of the real world, this one uses the end of the Age of the Dragon as Year 0 and counts forward.
A.D. was used during the Long Night and is considered a single age lasting around 3105 years. It is not broken down into sub-groups like the A.Y. System, so A.D. 2569 is actually the two thousnd, five hundred and sixty-ninth year After Dragon and not Year 569 of the Second Age.
Officially A.D stands for Age of Dawn, but After Dragon is a much more popular definition. Though in year 3105 of the Age of Dawn, the calender shifted once again. For a while, the new age would be called the Dawn of the Dragon, but eventually settled on The Age of the New Day and N.D. System.