Charles Ellicott 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)

I will add unto thy days fifteen years. —In the Jewish reckoning, fourteen years and a fraction of a year would count as fifteen years. With this very definite prediction, compare to Isaiah 7:8; Isaiah 23:15; Jeremiah 25:11–12.

And I will deliver thee ... —So that the Assyrians had not yet retired from the West. For the rest of the verse, see 2 Kings 19:34.