I always thought dating a hammer stone is pretty much based on context. I found them on the surface as well on digs. Not being an experienced knapper, I am not that familiar with the nuances that dictate age of a hammer stone. When I'm on a screen dig and one rolls across my view I make a mental note of what else came up at that depth at my table or other tables around me. Interestingly enough, finding one on a long term habitation or one that had many repeated habitations, could have been used many times over a few thousand years.