90:12 Teach us to number our days. An awareness of life’s brevity will lead to “a heart of wisdom.” This term probably defines the “fear that is your due” in the previous verse, for the “fear of the Lord is the beginning of wisdom” (Ps. 111:10; cf. Deut. 32:29). This is really the center of the psalm. At this point our poet thinks of death as the factor that helps the believer to realize “that God shows him the way to the true and ultimate meaning of life.”
12. “So teach us to number our days.” Instruct us to set store by time, mourning for that time past wherein we have wrought the will of the flesh, using diligently the time present, which is the accepted hour and the day of salvation, and reckoning the time which lieth in the future to be too uncertain to allow us safely to delay any gracious work or prayer. Numeration is a child’s exercise in arithmetic, but in order to number their days aright the best of men need the Lord’s teaching. We are more anxious to count the stars than our days, and yet the latter is by far more practical. “That we may apply our hearts unto wisdom.” Men are led by reflections upon the brevity of time to give their earnest attention to eternal things; they become humble as they look into the grave which is so soon to be their bed, their passions cool in the presence of mortality, and they yield themselves up to the dictates of unerring wisdom; but this is only the case when the Lord himself is the teacher; he alone can teach to real and lasting profit. Thus Moses prayed that the dispensations of justice might be sanctified in mercy. “The law is our schoolmaster to bring us to Christ,” when the Lord himself speaks by the law. It is most meet that the heart which will so soon cease to beat should while it moves be regulated by wisdom’s hand. A short life should be wisely spent. We have not enough time at our disposal to justify us in misspending a single quarter of an hour. Neither art we sure of enough of life to justify us in procrastinating for a moment. If we were wise in heart we should see this, but mere head wisdom will not guide us aright.
90:12 number our days. Evaluate the use of time in light of the brevity of life. heart of wisdom. Wisdom repudiates autonomy and focuses on the Lord’s sovereignty and revelation.
90:12 teach us to number our days. In view of the theme of the psalm, this refers especially to the ability to make the most of one’s days, since they are so few. The heart of wisdom would enable the faithful to live by the right priorities (cf. the “fear” of God, v. 11).
90:12 teach us to number our days A response to God’s power and wrath—emphasizing that people should pay attention to God’s ways each day and appreciate the life given to them.
a heart of wisdom Wisdom starts with being properly oriented to God.
90:12 The emphasis here is upon the frugal use of years and not upon the number of years. Usefulness surpasses longevity. The psalmist pleads for discernment in ordering his days.
90:12 To number our days is to measure the time left in life and make every day count. It is based on the recognition that life is short and God’s anger swift. Thus, the goal of such numbering is a heart of wisdom. Conscious of life’s brevity, we learn to make choices through which God can establish the work of our hands (v. 17), producing something valuable to those that follow us (v. 16) and honoring to God.
12. Teach us so to number our days. Some translate to the number of our days, which gives the same sense. As Moses perceived that what he had hitherto taught is not comprehended by the understandings of men until God shine upon them by his Spirit, he now sets himself to prayer. It indeed seems at first sight absurd to pray that we may know the number of our years. What? since even the strongest scarcely reach the age of fourscore years, is there any difficulty in reckoning up so small a sum? Children learn numbers as soon as they begin to prattle; and we do not need a teacher in arithmetic to enable us to count the length of a hundred upon our fingers. So much the fouler and more shameful is our stupidity in never comprehending the short term of our life. Even he who is most skilful in arithmetic, and who can precisely and accurately understand and investigate millions of millions, is nevertheless unable to count fourscore years in his own life. It is surely a monstrous thing that men can measure all distances without themselves, that they know how many feet the moon is distant from the centre of the earth, what space there is between the different planets; and, in short, that they can measure all the dimensions both of heaven and earth; while yet they cannot number threescore and ten years in their own case. It is therefore evident that Moses had good reason to beseech God for ability to perform what requires a wisdom which is very rare among mankind. The last clause of the verse is also worthy of special notice. By it he teaches us that we then truly apply our hearts to wisdom when we comprehend the shortness of human life. What can be a greater proof of madness than to ramble about without proposing to one’s self any end? True believers alone, who know the difference between this transitory state and a blessed eternity, for which they were created, know what ought to be the aim of their life. No man then can regulate his life with a settled mind, but he who, knowing the end of it, that is to say death itself, is led to consider the great purpose of man’s existence in this world, that he may aspire after the prize of the heavenly calling.
 MacArthur, J. F., Jr. (2006). The MacArthur study Bible: New American Standard Bible. (Ps 90:12). Nashville, TN: Thomas Nelson Publishers.
 Criswell, W. A., Patterson, P., Clendenen, E. R., Akin, D. L., Chamberlin, M., Patterson, D. K., & Pogue, J. (Eds.). (1991). Believer’s Study Bible (electronic ed., Ps 90:12). Nashville: Thomas Nelson.
 Cabal, T., Brand, C. O., Clendenen, E. R., Copan, P., Moreland, J. P., & Powell, D. (2007). The Apologetics Study Bible: Real Questions, Straight Answers, Stronger Faith (p. 867). Nashville, TN: Holman Bible Publishers.