I am not positive, but I saw in some timeline thing in social studies that he was born in 4bc and that when he was four, he started talking about God for the first time and that is when AD started.

AD is the abbriviation for some latin phrase that means "In The Year of Our Lord"

He didn't talk about God until that point so it wouldn't have actualy been "In the Year of Our Lord" until he started talking about God