Adam Clarke on Job 38:23

Reserved against the time of trouble - לעת צר leeth tsar, "to the season of strictness," i.e., the season when the earth is constringed or bound by the frost.

Against the day of battle and war? - Hailstones being often employed as instruments of God's displeasure against his enemies, and the enemies of his people. There is probably an allusion here to the plague of hail sent on the Egyptians. See Exo 9:23 (note), and the notes there, for more particulars concerning hailstones, remarkable showers of them, etc. There may be also a reference to Jos 10:10-11 (note), where a destructive shower of what are called hailstones fell upon the Canaanitish kings who fought against Israel. See the note there also.