Psalms 90:12

So teach us to number our days
Not merely to count them, how many they are, in an arithmetical way; there is no need of divine teachings for that; some few instructions from an arithmetician, and a moderate skill in arithmetic, will enable persons not only to count the years of their lives, but even how many days they have lived: nor is this to be understood of calculating or reckoning of time to come; no man can count the number of days he has to live; the number of his days, months, and years, is with the Lord; but is hid from him: the living know they shall die; but know not how long they shall live, and when they shall die: this the Lord teaches not, nor should we be solicitous to know: but rather the meaning of the petition is, that God would teach us to number our days, as if the present one was the last; for we cannot boast of tomorrow; we know not but this day, or night, our souls may be required of us: but the sense is, that God would teach us seriously to meditate on, and consider of, the shortness of our days; that they are but as a shadow, and there is no abiding; and the vanity and sinfulness of them, that so we may not desire to live here always; and the troubles and sorrows of them, which may serve to wean us from the world, and to observe how unprofitably we have spent them; which may put us upon redeeming time, and also to take notice of the goodness of God, that has followed us all our days, which may lead us to repentance, and engage us in the fear of God:

that we may apply our hearts unto wisdom;
to consider our latter end, and what will become of us hereafter; which is a branch of wisdom so to do; to seek the way of salvation by Christ; to seek to Christ, the wisdom of God, for it; to fear the Lord, which is the beginning of wisdom; and to walk circumspectly, not as fools, but as wise; to all which an application of the heart is necessary; for wisdom is to be sought for heartily, and with the whole heart: and to this divine teachings are requisite, as well as to number our days; for unless a man is taught of God, and by his Spirit convinced of sin, righteousness, and judgment, he will never be concerned, in good earnest, about a future state; nor inquire the way of salvation, nor heartily apply to Christ for it: he may number his days, and consider the shortness of them, and apply his heart to folly, and not wisdom; see ( Isaiah 22:21 ) .