What do B.C. and A.D. mean in relation to the year zero?
A.D. stands for Anno Domini, meaning "the year of our Lord," which on the Gregorian calendar is the year zero. B.C. means "before Christ," covering everything before that zero point, while A.D. covers everything after it as the year of our Lord.
Les teaches that A.D. comes from the Latin words Anno Domini—Anno giving us annual, Domini meaning Master or Lord—so A.D. means the year of our Lord. On the Gregorian calendar, that point is year zero. Everything back from zero is called before Christ, or B.C.; everything on the other side is called the year of our Lord, or A.D. He also notes the shift from B.C. to B.C.E. and from A.D. to C.E., or Common Era, which he says removes Christ from the picture.