Biruni is obviously using the past tense. That's all I can gather from the sentence. So it can be any time before his time (which would be pre-Turkic anyway). But then again, to answer your question, we'd have to read the entire passage of the Biruni quote to investigate the full chronological context. It will take me a while (after May the soonest) before I can go to Austin (where they have a copy of the Athar-al Baqiyah) to shoot a picture of the page and post it on the talk page for all to view. I'm currently stuck with commitments to attend to.