Wouldn't it make more sense to mimic the timeline structure of the real world? Like so:
The founding of Ansus would be 0(Adventum). Which would mean that Anno Tenebrarum begins at 50,000 Post Adventum. 

So i'll use Daen's time of legend again as example again. 
30,000 before present. = 20,000 Post Adventum, or 30,000 before Anno Tenebrarum