Jamieson, Fausset and Brown on Job 17:12

12. They--namely, "my friends."

change the night into day--that is, would try to persuade me of the change of my misery into joy, which is impossible [Umbreit] (Job 11:17); (but) the light of prosperity (could it be enjoyed) would be short because of the darkness of adversity. Or better for "short," the Hebrew "near"; "and the light of new prosperity should be near in the face of (before) the darkness of death"; that is, they would persuade me that light is near, even though darkness approaches.