OSES was used by God to author this great psalm. It is a meditation about time. First of all, let us all be reminded that God existed before time existed. And, when time has expired, God will still exist in all of His immutable reality. Not only this, but God also perceives time very differently than we do (v. 4). Because of His omniscience, wisdom, power, sovereignty and immutability, He knows every detail about the past and about the future. He sees them as clearly as if it were the present (Is. 46:9-10). In fact, He knows the future better than we know the past or present.
I suppose that if it had not been for the fall in Eden, time would be all but irrelevant. Without sin, there would be no curse. Without the curse, there would be no limitations, deaths or conclusions. Would time have any bearing on us if we weren't going to run out of it? Time, that is, history has been dominated and even castrated by evil (Ephesians 5:16). If humanity hadn't rebelled against God, our existence would not be hollow and empty and evil. But, we did (and do) rebel and hence our world is abundantly given over to vanity (Ecclesiastes 1:14). In the words of Moses, "All our days are passed away in Thy wrath..." (Psalm 90:8).
For us, time is indeed a gift. None of us know when we will run out of time permanently. It is (in a way) a more valuable asset than money, property, intelligence or many other of our prized resources. Yet, it is limited, and we would all be wise to do well in the time that has been allotted to us. The time will come eventually when it will be "time" to answer to God for how we spent our time.