John Calvin Commentary Genesis 8:3

"and the waters returned from off the earth continually: and after the end of a hundred and fifty days the waters decreased." — Genesis 8:3 (ASV)

And after the end of the hundred and fifty days. Some think that the whole time, from the beginning of the deluge to the abatement of the waters, is noted here; and so they include the forty days in which Moses relates that there was continuous rain. But I make this distinction: until the fortieth day, the waters rose gradually with fresh additions; then they remained in nearly the same state for one hundred and fifty days, for both computations make the period a little more than six and a half months.

And Moses says that, about the end of the seventh month, the diminution of the waters appeared to be such that the ark settled upon the highest summit of a mountain, or touched some ground. And by this lengthened space of time, the Lord would show more plainly that the dreadful desolation of the world had not fallen upon it accidentally, but was a remarkable proof of his judgment; while the deliverance of Noah was a magnificent work of his grace, and worthy of everlasting remembrance.

If, however, we count the seventh month from the beginning of the year (as some do), and not from the time that Noah entered the ark, the subsidence of which Moses speaks took place earlier, namely, as soon as the ark had floated five months. If this second opinion is accepted, there will be the same reckoning of ten months; for the sense will be that in the eighth month after the commencement of the deluge, the tops of the mountains appeared.

Concerning the name Ararat, I follow the most widely accepted opinion. And I do not see why some should deny it to be Armenian, whose mountains are declared, by ancient authors, almost with one consent, to be the highest. The Chaldean paraphrase also points out the particular part, which it calls mountains of Cardu, which others call Cardueni. But whether what Josephus handed down concerning the fragments of the ark found there in his time is true (remnants of which, Jerome says, remained into his own age), I leave undecided.