The Gregorian calendar started counting progressive years after the birth of Jesus. Years before his birth are counted in reverse.
So if it's 2016 and the world is supposed to be millions of years old, how does this work? Why did we switch from B.C. to AD?
2016 has nothing at all to do with the age of the Earth. The Earth existed for billions of years before man appeared, and man was around for many thousands of years before he kept calendars. Modern society currently observes the Gregorian (also Western, or Christian) calendar, which was/is counted from a 6th century estimate of the date of Christ's birth.
BC is meant as the period "Before Christ"; AD stands for Anno Domini- latin for "in the year of the Lord".
These terms have been largely replaced by the more secular BCE -Before the Common Era; and, CE - Common Era