Albert Barnes Commentary Psalms 90:12

"So teach us to number our days, That we may get us a heart of wisdom." — Psalms 90:12 (ASV)

So teach us to number our days - literally, To number our days make us know, and we will bring a heart of wisdom. The prayer is that God would instruct us to estimate our days correctly: their number; the rapidity with which they pass away; the liability to be cut down; the certainty that they must soon come to an end; their bearing on the future state of being.

That we may apply our hearts unto wisdom - Margin, “Cause to come.” We will bring, or cause to come, a heart of wisdom. By taking a proper account of life, so that we may bring to it a heart truly wise, or act wisely in view of these facts. The prayer is that God would enable us to form such an estimate of life that we will be truly wise, so that we may be able to act as if we saw the whole of life, or as we should act if we saw its end.

God sees the end—the time, the manner, and the circumstances in which life will close. Although he has wisely hidden that from us, he can still enable us to act as if we saw it for ourselves. He can enable us to have the same aims before us and to make as much of life as if we saw when and how it would close. If anyone knew when, and where, and how he was to die, it might be presumed that this would have an important influence on him in forming his plans and on his general manner of life. The prayer is that God would enable us to act as if we had such a view.