Question: how do years bc work?
Answer: In the modern calendar, we label all years with B.C. (before Christ) or A.D. (anno domini, or "in the year of our lord"). There is no "zero" year -- in this system, the year Christ was born is 1 A.D., and the year preceding it is 1 B.C. ... Besides B.C. and A.D., some people use B.C.E.