Adam Clarke on Psalms 102:13

Thou shalt arise, and have mercy upon Zion - While he is humbled at the footstool of mercy, and earnestly praying for mercy, an answer of peace is given; he is assured, not only that they shall be delivered, but that the time of deliverance is at hand. The set time - the seventy years predicted by Jeremiah, was ended; and God gave him to see that he was ever mindful of his promises.