AD

AD or A.D. was the abbreviated form of Anno Domini. The AD epoch began either on or one year after 0 and counted upward. In mathematical terms, it was the positive counterpart to the previous epoch, BC. Adric once referred to this dating method when he informed Tegan that the TARDIS had arrived in the year "2526 in the time scale you call Anno Domini." (DW: Earthshock)

The question of whether the year 0 was itself a part of the era was central to the disputed question of whether the year 2000 was a part of the 21st century or the 20th century. Different people, like the noted chronologist Professor Wagg (DW: Doctor Who) and even the Eighth Doctor himself (DWN: Doctor Who - The Novel of the Film) believed AD did start with year 0, because they thought 2000 was the start of the 21st century. The Sixth Doctor (MA: Millennial Rites) and Dave Young (EDA: Escape Velocity) strongly felt otherwise.

Behind the scenes
An inherently Christian designation, AD is not generally used in calendars of the traditionally non-Christian world. Consequently, some people use BCE (meaning Before Common Era) for BC. CE (meaning Common Era) as a substitute for AD. These largely cosmetic replacements are thought to avoid religious offence. However, Doctor Who fiction has typically remained loyal to the BC/AD convention. Even as late as the RTD and Moffat eras, dialogue and on-screen graphics prefer the Christian convention. (DW: The Fires of Pompeii, The Pandorica Opens, A Good Man Goes to War)