The B.G.A. System was not formally introduced until the early Second Age. Prior to its introduction it was often used informally by historians, although consensus of the exact beginning of the Golden Age had not been established.
All B.G.A. dates count backwards from A.Y. 10, or Year 0 of the Golden Age (See A.Y. System). so B.G.A. 1000 would be 1000 years before Year 0 of the Golden Age.
B.G.A. dates are sometimes written as '(year) B.G.A.' or "years before Golden Age". This notation was most common during the Second and Third Ages when most work on the Mage Wars was being composed. Written histories from that time were transcribed and translated, and entered into the new system.
Writing the date after B.G.A. did not become common until the Fourth Age, following a lengthy lull in archeologicial studies. Modern scholars of the time felt using the notation B.G.A. (Year) was more in line with the A.Y. (Year) system then in common use.
Both notations are acceptable.