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)
The Time Lords' Celestial Intervention Agency acknowledged that the Earth year 0 AD corresponded with the year 70,708 of their TL time scale. (PROSE: A Sourcebook for Field Agents)
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)
According to one account, the First Doctor said there was no such year; he once set the TARDIS' space-time clock so that if something was about to go wrong, the dials would set themselves to read "AD 0000" and the clock would isolate itself from the circuit, thus saving him the trouble of having to repair it. (PROSE: The Space Museum)
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.