The B.F. or Before Fall System is used to fix dates in Antiquity, counting backwards from the Fall of Roads. For example, the era known as 'Classic Antiquity' lasted approximately 10,000 years, so the date of transition between the Early and Classic periods would be B.F. 10,000.
However, the exact date of the Fall is unknown, making it nearly impossible to fix dates in antiquity. The fall is believed to have occured in roughly 4700 B.G.A., however the margin for this is completely unknown. The Age of Darkness is believed to have lasted roughly 1000 years, with the Mage Wars begining in 3700 B.G.A.(about).
If the Age of Darkness can be assumed to have lasted roughly 1000 years, then the 4700 B.G.A. date is most likely. Even then, dates from that era cannot be confirmed with any kind of certainty. Because everything that is known of Antiquity is based on speculation, no dates within it are considered to be at all accurate.