Calvin's Commentaries, Vol. 27: Joel, Amos, Obadiah, tr. by John King, [1847-50], at sacred-texts.com
He shows himself the time when he began to discharge his office of a teacher; but it does not appear how long he prophesied. The Jews indeed, think that his course was long; he continued his office, as they write, under four kings. But he mentions here only the reigns of Uzziah and Jeroboam. His purpose was to mark the time when he began to execute his office of a Prophet, but not to express how long he labored for God in that office; and why he mentions only the beginning, we shall in its proper place notice. It is, indeed, certain, that he commenced his work under king Uzziah, and under king Jeroboam: and this also is to be noticed, that he was appointed a Prophet to the kingdom of Israel. For though he arose from the tribe of Judah, yet the Lord, as we shall see set him over the kingdom of Israel. He sometimes turns his discourse to the tribe of Judah, but only, as it were, accidentally, and as occasion led him; for he mainly addressed the Ten Tribes. I now come to his words.