Jamieson, Fausset and Brown on Ecclesiastes 3:17

17. Solution of it. There is a coming judgment in which God will vindicate His righteous ways. The sinner's "time" of his unrighteous "work" is short. God also has His "time" and "work" of judgment; and, meanwhile, is overruling, for good at last, what seems now dark. Man cannot now "find out" the plan of God's ways (Ec 3:11; Psa 97:2). If judgment instantly followed every sin, there would be no scope for free will, faith, and perseverance of saints in spite of difficulties. The previous darkness will make the light at last the more glorious.

there--(Job 3:17-19) in eternity, in the presence of the Divine Judge, opposed to the "there," in the human place of judgment (Ec 3:16): so "from thence" (Ge 49:24).