John Calvin Commentary Isaiah 23:15

"And it shall come to pass in that day, that Tyre shall be forgotten seventy years, according to the days of one king: after the end of seventy years it shall be unto Tyre as in the song of the harlot." — Isaiah 23:15 (ASV)

And it shall come to pass in that day. After having spoken of the taking of Tyre, he next declares how long her calamity shall endure. It happens that cities which have been ruined are suddenly restored and regain their former position; but the Prophet testifies that this city will be desolate and ruinous for seventy years. By being forgotten he means that there will be no merchandise, because she will not have the ordinary course of trade.

According to the days of one king. Some think that the days of one king relate to David, but that is exceedingly frivolous, for “the days of a king” stand for the age of a man, in the same manner as the age of a man is shown by the Psalmist to be generally limited to seventy years (Psalms 90:10). But why did he mention “a king” rather than any other man? It was because Tyre had a king and reckoned time by the life of a king. This contributed greatly to establish the certainty of the prediction, for the Prophet could not have ascertained it by human conjectures.

Tyre shall have a song like that of a harlot. By “the song of a harlot” he employs a beautiful comparison to denote merchandise; not that in itself it ought to be condemned, for it is useful and necessary to a commonwealth, but he alludes to the fraud and dishonesty with which it frequently abounds, so that it may justly be compared to the occupation of a harlot.