Keeping time: The history, origin and meanings of B.C. and A.D.

A close-up of a gilded astronomical clock on a clock tower that reads "Anno Domini" in the center.

When reading historical texts or delving into archaeology stories, it's common to see "A.D." and "B.C." added to dates. These common terms divide time before and after the birth of Jesus Christ, though they may also be swapped out for the more-secular "C.E" and "B.C.E." But what do "A.D" and "B.C." actually mean, and where did they come from?