Mat
Well, obviously he didn't know! But from the scripture we are discussing it is clear that Paul believed that he would be among the living on the day of the Lord. It is also clear that he made his contemporary generation believe the Lords day would be in their lifetime. So much for God's inspiration eh?
Paul and his "contemporary generation" and others, down through years believed that the Day of the Lord COULD (not would) come during their lifetimes.