John Calvin Commentary Isaiah 38:8

"behold, I will cause the shadow on the steps, which is gone down on the dial of Ahaz with the sun, to return backward ten steps. So the sun returned ten steps on the dial whereon it was gone down." — Isaiah 38:8 (ASV)

Look, I bring back the shadow of degrees. The sign that is given here to Hezekiah is the shadow on the sundial going back, along with the sun, ten degrees by which it had already gone up—that is, had advanced above the horizon. And this sign bears a resemblance to the event itself, as all other signs generally do; for it is as if God had said, “As it is in my power to change the hours of the day and to make the sun go backward, so it is in my power to lengthen your life.”

Regarding the shadow not going back as many degrees as the years added to his life, this was impossible because there were not more than twelve degrees on the sundial. The day was divided by these degrees into twelve hours, which were longer or shorter according to the change of the season. Therefore, we do not need to be uneasy about the number; it is enough that there is a clear correspondence and resemblance.

On the sundial of Ahaz. Here the Jews, according to their custom, invent fables and devise a story that the day on which Ahaz died was shorter than ten hours, and that what God had justly inflicted on him as a punishment for his sins was reversed for Hezekiah’s benefit, because the shortening of one day was the lengthening of another. However, there is no historical account of this, and it entirely lacks not only evidence but also probability; nor is anything said here about the death of Ahaz or about the change that took place when he died, but only about the sundial that he had made.