I regularly caused consternation among the nuns at my Catholic grade school. During one religion class, we were being taught what "AD" and "BC" meant for calendar dates. I raised my hand and asked how the people back then knew what to call it. The teacher reiterated, "it's After Death and Before Christ, Jeffrey. Now moving on..." And I said: no, no, back in 100 BC, how did they know to count the years backwards? Did they know Christ was going to show up in 100 years??
The class erupted, of course. This was a head-scratcher. Nobody really had the presence of mind to tell me that they probably called the years something else back then. I guess it's hard to see out of such an ingrained way of looking at things.
And now I have my answer: over at The Renaissance Mathematicus, thonyc tells us that the use of "AD" started around the year 525, and "BC" came into use in the 1600s. Apparently beforehand, they had numbered the years by the reigns of the popes.
And that's your history lesson for the day.