who, when, and why was the dates designated BC and AC?

Answer 1

This is what I found, from a rather interesting site:

"The abbreviation A.D. stands for Anno Domini (Latin) - "in the year of the Lord", meaning the year(s) since Christ's birth. It wasn't adopted until around 386 A.D. when a Catholic monk decided to find out how many years had past since Christ's birth. Sadly he was 7 years off. So instead of Christ being born in 1 A.D. (because there is no 0 A.D.), Jesus was actually born in 7 B.C. What I mean is if the monk had found the correct year, this year wouldn't have been 2007, but 2014.

The Christian origin of the designation of years as "B.C." (Before Christ) and "A.D." (Anno Domini, Latin for "in the year of the Lord") has led some people to favor a less culturally specific designation. "C.E." stands for "the Common Era," and "B.C.E." for "Before the Common Era." In practice, the terms are perfect synonyms for "B.C." and "A.D.""
http://www.abdicate.net/cal.aspx

Just found something else:
http://www.calendar-origins.com/year-origin.html
http://www.12x30.net/faq.html

Answer 2

BC - Before Christ
AC - After Christ

For reasons of political correctness it is now:
BC is now BCE = Before Common Era
AC is now CD = Common Era

Answer 3

BC= Before Christ (before His birth)
AC= After Christ (after His birth)

Answer 4

AC? You must go to public school.