Charles Ellicott Commentary Isaiah 38:5

"Go, and say to Hezekiah, Thus saith Jehovah, the God of David thy father, I have heard thy prayer, I have seen thy tears: behold, I will add unto thy days fifteen years." — Isaiah 38:5 (ASV)

Fifteen years. —The words fix the date of the illness, taking the received chronology, as 713 B.C. The next verse shows that there was danger at the time to be feared from Assyria, but does not necessarily refer to Sennacherib’s invasion. Sargon’s attack (Isaiah 20:1) may have caused a general alarm.