AD

ADor A.D. was the abbreviated form of Anno Domini. The AD epoch followed either year 0, or the year 1 BC 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." (TV: Earthshock)

Year Zero
The question of whether the year 0 existed was central to the dispute over whether the year 2000 was the last year of the second millennium or the first year of the third millennium. Different people, like the noted chronologist Professor Wagg (TV: Doctor Who) and even the Eighth Doctor himself (PROSE: The Novel of the Film) believed AD did start with year 0, because they thought 2000 was the start of the new millennium. The Sixth Doctor (PROSE: Millennial Rites) and Dave Young (PROSE: Escape Velocity) strongly felt otherwise.

Year of our Lord
The "year of our Lord" was a labeling applied to years which was used in Britain.

In 1638, a mathematician calculated that the Nemesis would return in the "year of our Lord" 1988. (TV: Silver Nemesis)

A young boy informed the Tenth Doctor that he was in the "year of our Lord" 1851. (TV: The Next Doctor)

In 1879, Robert MacLeish noted that a shooting star landed in the "year of our Lord" 1540. (TV: Tooth and Claw)

In 1913, John Smith experienced dreams which took place in the future "year of our Lord" 2007. (TV: Human Nature)

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 and 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. (TV: The Fires of Pompeii, The Pandorica Opens, A Good Man Goes to War, Robot of Sherwood)
 * In the real world, there is no year zero, so 1 BC is immediately followed by 1 AD. Doctor Who fiction, however, is unclear as to this point.
 * On this wiki, templates and categories, both of which use, or are used in, mathematical formulae, assume the presence of a year zero, since the fiction of the DWU allows it. Hence, we deem that the year 2000 is in the 21st century.