[@Darkspleen]
Its like moving from BC to AD, In between BC and AD, there is 0 which would signify an event that would transit from those two classifications. In our case, the death of Christ (or after Common Era) and in the Star Wars Universe it would be the Battle of Yavin. So in essence, we are playing characters in the past and are moving forward in time to point '0'. 

From the wiki, it explains it here...
[quote]
...Calendar eras used the Battle of Yavin as its epoch, or "year zero." BBY stands for Before the Battle of Yavin, and ABY stands for After the Battle of Yavin.[/quote]