Adam Clarke on Job 17:12

They change the night into day - These purposes and thoughts are so very gloomy, that they change day into night.

The light is short because of darkness - אור קרוב מפני חשך or karob mippeney choshek, "The light is near from the face of darkness." I have scarcely any light: what is called light is so near akin to darkness, that it is scarcely severed from it. There is either no light, or merely such as is sufficient to render darkness visible. A fine picture of the state of his mind - he was generally in darkness; but had occasional gleams of hope.