The king of Assyria in 714 and 713 B.C. was Sargon (B.C. 721-705). If then the Biblical and Assyrian chronologies which agree exactly in the year of the taking of Samaria (721 B.C.), are to be depended on, the king of Assyria here must have been Sargon. It may be conjectured that he had taken offence at something in the conduct of Hezekiah, and have threatened Jerusalem about this time (compare Isaiah 20:6). There is, however, no evidence of actual hostilities between Judaea and Assyria in Sargon's reign.

I will add unto thy days fifteen years - This is the first and only man who was ever informed of the term of his life. And was this a privilege! Surely no. If Hezekiah was attached to life, as he appears to have been, how must his mind be affected to mark the sinking years! He knew he was to die at the end of fifteen years; and how must he feel at the end of every year, when he saw that so much was cut off from life? He must necessarily feel a thousand deaths in fearing one. I believe there would be nothing wanting to complete the misery of men, except the place of torment, were they informed of the precise time in which their lives must terminate. God, in his abundant mercy, has hidden this from their eyes.

And I will add unto thy days fifteen years,.... See Gill on Isaiah 38:5.

and I will deliver thee, 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 mine 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.

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.

20:6 Fifteen years - We have not an instance of any other, who was told before - hand just how long, he should live. God has wisely kept us at uncertainties, that we may be always ready.

20:1-11 Hezekiah was sick unto death, in the same year in which the king of Assyria besieged Jerusalem. A warning to prepare for death was brought to Hezekiah by Isaiah. Prayer is one of the best preparations for death, because by it we fetch in strength and grace from God, to enable us to finish well. He wept sorely: some gather from hence that he was unwilling to die; it is in the nature of man to dread the separation of soul and body. There was also something peculiar in Hezekiah's case; he was now in the midst of his usefulness. Let Hezekiah's prayer, see Isa 38. interpret his tears; in that is nothing which is like his having been under that fear of death, which has bondage or torment. Hezekiah's piety made his sick-bed easy. O Lord, remember now; he does not speak as if God needed to be put in mind of any thing by us; nor, as if the reward might be demanded as due; it is Christ's righteousness only that is the purchase of mercy and grace. Hezekiah does not pray, Lord, spare me; but, Lord, remember me; whether I live or die, let me be thine. God always hears the prayers of the broken in heart, and will give health, length of days, and temporal deliverances, as much and as long as is truly good for them. Means were to be used for Hezekiah's recovery; yet, considering to what a height the disease was come, and how suddenly it was checked, the cure was miraculous. It is our duty, when sick, to use such means as are proper to help nature, else we do not trust God, but tempt him. For the confirmation of his faith, the shadow of the sun was carried back, and the light was continued longer than usual, in a miraculous manner. This work of wonder shows the power of God in heaven as well as on earth, the great notice he takes of prayer, and the great favour he bears to his chosen.