"I have seen the business that God has given to the sons of men to be busy with. He has made everything beautiful in its time; also he has put eternity into man's mind, yet so that he cannot find out what God has done from the beginning to the end" (Ecclesiastes 3:10-11).
This is, indeed, a bit confusing. I think what the author is saying is that while we (mankind) understand the fact of eternity, we can't see eternity from the perspective of the Divine. Our human perspective is always more limited. The only moment of eternity we can truly touch and experience is the very Present moment. So that's where our spiritual focus needs to be.