You can't. BC dates are BC dates and cannot be converted into AD dates.
Besides, which calendar would you use? The modern Gregorian Calender was introduced by Pope Gregory XIII 1582. The Julian Calendar, which preceded it, was introduced by Julius Caesar in 45 BC (but even then, its leap-year calculations weren't always observed). Before that, the Roman calendar was such a mess we often can't be sure what the date was when a given event occurred (or how that date might relate to either the Julian or Gregorian calendars). And that only accounts for the 'West'; it says nothing of all the other calendars in use over the millennia.
For further details, see:
http://www.tondering.dk/claus/calendar.html