Moses’ response to contemplating the truths contained in Ps. 90:1-11 is expressed at the beginning of v.12—Teach us or make known to us how to count our days. He’s not asking for a divine math tutor, but instead, as the NET translates—to rightly consider our mortality. Or in the words of Ps. 39:4 “Lord, make me to know my end And what is the extent of my days; Let me know how transient I am.
What will that accomplish? Everyone knows that their life on earth is limited, but that knowledge does not automatically lead to godliness. You’ve probably heard these mantras—”Life is short; play hard.” Or “Life is too short to not pursue maximum happiness.” Or “Life is short, so enjoy it to the fullest.” Those recognitions of the transiency of life lead to selfish idolatry instead of worshipful allegiance to the Lord. This means that there’s a way to know about the transiency of life and yet still store up increasing judgement from God.
On the other hand, understanding that life is short from a biblical perspective is always manifested in how one lives. Look at the 2nd line of v.12—That we may be caused to gain a heart of wisdom or have a heart of skillful living (Prov. 23:12). As we mentioned many times, the word heart in Scripture does not merely refer to the hub of our emotional life. It entails motives, desires and even thoughts. So if one can gain skillful living as a result of bringing one’s motives, desires and thoughts under God’s will, then and only then will that person benefit greatly from knowing the brevity of life.