Actually, if the geneologies in the Bible establish anything at all, it would be that man has been present on the earth for 8 to 10 thousand years.
There is considerable debate on how long the five "days" of creation that went on before that actually lasted. For instance, how long was the night before God said "Let there be light."? The Bible says that there was evening, and there was morning, one day. So, do you think that He started time exactly 12 hours before He created light?
It doesn't matter when he started time or how long he was in existence before time began. What's the point? There was nothing until time began, except God, and that is where this discussion is based.