Adam Clarke on Jeremiah 28:17

So Hananiah - died the same year in the seventh month - The prophecy was delivered in the fifth month, (Jer 28:1), and Hananiah died in the seventh month. And thus God, in mercy, gave him about two months, in which he might prepare to meet his Judge. Here, then the true prophet was demonstrated, and the false prophet detected. The death of Hananiah thus predicted, was God's seal to the words of his prophet; and must have gained his other predictions great credit among the people.