And this shall be a sign unto thee from the Lord. It was the day of the free offering of "signs" by God to those whom his providence had placed at the head of his people. Ahaz had been offered a sign (Isaiah 7:11), but had refused the offer made him (Isaiah 7:12); the Lord had then "himself" given him a sign."
Hezekiah received a sign to assure him of the complete discomfiture of Sennacherib (Isaiah 37:30); an offer was here made him of a sign of a peculiar kind, and it was offered under peculiar conditions.
We learn from 2 Kings that a choice was submitted to him—he was to determine whether time, as measured by a certain timepiece or clock, which was known as "the dial of Ahaz," should make a sudden leap forward—the shadow advancing ten degrees upon the dial (2 Kings 20:9), or whether it should retire backwards, the shadow upon the same dial receding ten degrees.
Hezekiah determined in favour of the latter sign, from its appearing to him the more difficult of accomplishment; and on his declaring his decision, the shadow receded to the prescribed distance. Time was rolled backward, or at any rate appeared to be rolled backward; and the king, seeing so great a miracle, accepted without hesitation the further predictions that had been made to him.
The Lord will do this thing that he hath spoken. By the nexus of this verse with the preceding, it would naturally be concluded that "the thing" to be done was the defence of Jerusalem; but verse 22, which belongs properly to this part of the narrative, shows the contrary.
Hezekiah had asked for a sign" that he should go up to the house of the Lord."