Calvin's Take on Jonah (4:5)

It may be here doubted whether Jonah had waited till the forty days had passed, and whether that time had arrived; for if we say that he went out of the city before the fortieth day, another question arises, how could he have known what would be? for we have not yet found that he had been informed by any oracular communication. But the words which we have noticed intimate that it was then known by the event itself, that God had spared the city from destruction; for in the last lecture it was said, that God had repented of the evil he had declared and had not done it. It hence appears that Jonah had not gone out of the city until the forty days had passed. But there comes again another question, what need had he to sit near the city, for it was evident enough that the purpose of God had changed, or at least that the sentence Jonah had pronounced was changed? he ought not then to have seated himself near the city as though he was doubtful.
But I am inclined to adopt the conjecture, that Jonah went out after the fortieth day, for the words seem to countenance it. With regard to the question, why he yet doubted the event, when time seemed to have proved it, the answer may be readily given: though indeed the forty days had passed, yet Jonah stood as it were perplexed, because he could not as yet feel assured that what he had before proclaimed according to God's command would be without its effect. I therefore doubt not but that Jonah was held perplexed by this thought, "Thou hast declared nothing rashly; how can it then be, that what God wished to be proclaimed by his own command and in his own name, should be now in vain, with no corresponding effect?" Since then Jonah had respect to God's command, he could not immediately extricate himself from his doubts. This then was the cause why he sat waiting: it was, because he thought that though God's vengeance was suspended, his preaching would not yet be in vain, but that the ruin of the city was at hand. This therefore was the reason why he still waited after the prefixed time, as though the event was still doubtful.
Now that this may be more evident, let us bear in mind that the purpose of God was hidden, so that Jonah understood not all the parts of his vocation. God, then, when he threatened ruin to the Ninevites, designed to speak conditionally: for what could have been the benefit of the word, unless this condition was added, -- that the Ninevites, if they repented, should be saved? There would otherwise have been no need of a Prophet; the Lord might have executed the judgment which the Ninevites deserved, had he not intended to regard their salvation. If any one objects by saying that a preacher was sent to render them inexcusable, -- this would have been unusual; for God had executed all his other judgments without any previous denunciation, I mean, with regard to heathen nations: it was the peculiar privilege of the Church that the Prophets ever denounced the punishments which were at hand; but to other nations God made it known that he was their Judge, though he did not send Prophets to warn them. There was then included a condition, with regard to God's purpose, when he commanded the Ninevites to be terrified by so express a declaration. But Jonah was, so to speak, too literal a teacher; for he did not include what he ought to have done, -- that there was room for repentance, and that the city would be saved, if the Ninevites repented of their wickedness. Since then Jonah had learned only one half of his office, it is no wonder that his mind was still in doubt, and could not feel assured as to the issue; for he had nothing but the event, God had not yet made known to him what he would do.