Going into reverse crossing the BCE/CE divide, causes minor problems.
If you add a BCE date to a CE date, SUBTRACT 1 from the result to get the time in years.
2 BCE + 2 CE= 4-
To cross BCE to CE or CE to BCE a given number of years, subtract present date from the year number then ADD 1.
3 BCE forward 5 years;
Use the BCE CE boxes to to prove to yourself by counting.
Convert any minus figure to positive -