John Calvin Commentary Psalms 61:6

"Thou wilt prolong the king`s life; His years shall be as many generations." — Psalms 61:6 (ASV)

You shall add days upon days to the king etc. David cannot be considered as using these words of congratulation with an exclusive reference to himself. It is true that he lived to an extreme old age, and died full of days, leaving the kingdom in a settled condition and in the hands of his son, who succeeded him; but he did not exceed the period of one man’s life, and the greater part of it was spent in continual dangers and anxieties.

There can be no doubt, therefore, that the series of years, and even ages, of which he speaks, extends prospectively to the coming of Christ, since the very condition of the kingdom, as I have often remarked, was that God maintained them as one people under one head or, when scattered, united them again. The same succession still continues in relation to us. Christ must be viewed as living in his members to the end of the world. To this Isaiah alludes when he says, Who shall declare his generation or age? — words in which he predicts that the Church would survive through all ages, despite the incessant danger of destruction to which it is exposed through the attacks of its enemies and the many storms assailing it. So here David foretells the uninterrupted succession of the kingdom down to the time of Christ.