John Gill Commentary Isaiah 14:28

"In the year that king Ahaz died was this burden." — Isaiah 14:28 (ASV)

In the year that King Ahaz died was this burden. The following heavy prophecy, concerning the destruction of the Philistines; whether it was delivered out before or after his death is not certain. Here some begin the "fifteenth" chapter (Isaiah 15:1), and not improperly; henceforward prophecies are delivered out under another reign, as before under Uzziah, Jotham, and Ahaz, now under Hezekiah. This, according to Bishop Usher, was A. M. 3278 and before the Christian era 726.