Hezekiah's illness and fifteen more years
Isaiah told the dying Hezekiah to put his house in order, but when Hezekiah wept and prayed, God added fifteen years to his life and made the shadow go back ten steps.
“Turn back, and tell Hezekiah the prince of my people, ‘Yahweh, the God of David your father, says, “I have heard your prayer. I have seen your tears. Behold, I will heal you. On the third day, you will go up to Yahweh’s house.”
2 Kings 20:5 (World English Bible)
Hezekiah's prayer was answered almost before Isaiah had left the palace courtyard. Isaiah 38 preserves the song Hezekiah wrote after he recovered.
Dating notes
The added 15 years (2 Kings 20:6) run to Hezekiah's death in 698 BC, which places the illness in 713 BC, the same year as the invasion (“in those days,” 20:1).