John Gill

The Lord knoweth the days of the upright,.... Not only how long they shall live, and so fill up their days, the number of them, as Aben Ezra interprets it; but the nature of their days, whether prosperous or adverse; and causes both to work together for their good; and he knows the work of their days, as Jarchi explains it, the actions done by them in faith and love, and to his honour and glory, and which he approves and accepts of in Christ; see Psa 1:6; and their inheritance shall be for ever; in heaven, which is an eternal and never fading one, Heb 9:15; and therefore they ought not to fret and be envious.