0

According to the Eighth Doctor, "round about 0 BC, AD", the first Christmas took place. He went to attend it with Leonardo da Vinci. (AUDIO: Relative Dimensions) Indeed, both he, news stations in San Francisco, at least a significant portion of the staff of Walker General Hospital, and noted scholars like Professor Wagg believed that the year 2000 started a new millennium, thus indicating that there was a year zero. (TV: Doctor Who)

Others felt that year zero was a fiction. Both the Sixth Doctor and Anji Kapoor's ex-boyfriend Dave Young passionately argued against the existence of a year zero. (PROSE: Millennium Shock, Escape Velocity)

Behind the scenes
In the real world, the Christian calendar does not offer a year zero. Time simply progresses directly from 1 BC to 1 AD. Nevertheless, because a case for the existence of year zero can easily be built from significant DWU sources — and because it makes mathematical templates easy to construct here — the Tardis community chooses to believe the Eighth Doctor's view on the matter over the Sixth's.