Carolyn,
  Your idea sounds good, and would be true except for one little detail, and
that is there was no year designated as "zero." The dates went like this: BC
2 -- BC 1 -- AD 1 -- AD 2, etc. Therefore when adding or subtracting from
B.C. to A.D. you can easily see that between BC 2 and BC 1 is one year, and
from BC 1 to AD 1 is one year, and from AD 1 to AD 2 is one year. Therefore,
when you get to AD 2 you actually have three years. This is why chronologers
always add one year to bring the time accurate.
   Your mathematical concept of plus numbers and minus numbers has a zero in
it. However, the concept of zero was not used in mathematics in ancient
times -- it is a somewhat modern invention.

Bonnie