John Gill Commentary 2 Kings 20:6

"And I will add unto thy days fifteen years; and I will deliver thee and this city out of the hand of the king of Assyria; and I will defend this city for mine own sake, and for my servant David`s sake." — 2 Kings 20:6 (ASV)

And I will add to your days fifteen years,....See Gill on "Isaiah 38:5"

and I will deliver you, and this city, out of the hand of the king of Assyria; by which it appears that this sickness and recovery were before the destruction of the Assyrian army:

and I will defend this city for my own sake, and for my servant David's sake: for the sake of his honour and glory in the temple, and the service of it, that were in Jerusalem, and for the sake of his promise to David and his seed.