AD

Abbreviated form of Anno Domini or "After Christ", commonly used in the English-speaking world. The AD epoch begins with the year traditionally designated as that of the birth of Jesus of Nazareth. In "Earthshock," Adric refers to this dating method when he informs Tegan that the TARDIS has arrived in the year, "2526 in the time scale you call Anno Domini."

See also: BC, BCE, CE, Humanian Era, TL

For more detailed background on Anno Dominii, see the Wikipedia article on that topic.