Burkitt::

8 But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day. 9 The Lord is not slack concerning his promise, (as some men count slackness,) but his long-suffering to us-ward, not willing that any should perish, but that all should come to repentance.

Our apostle here answers the cavil and objection of the fore-mentioned scoffers, namely. That if Christ intends to come to judgment, why does he so long defer his coming? To this our apostle replies, 1. By assuring Them that this delay ought not to be judged according to our sense and apprehension of things, tor God does not measure time as we do; but a thousand years, which seem so long to us, are but a day, yea, but a moment, to him who is eternal, and inhabits eternity. To the eternity of God no finite duration bears any proportion, to eternity all time is equally short; God does not measure time by our pole, nor cast up years by our arithmetic. 2. He assures them farther, that God’s delay of judgment did not proceed from slackness, but from divine patience and goodness. He delays his coming, on purpose to give men time to repent, and by repentance to prevent their own eternal ruin. Learn hence, 1. That God’s delay of judgment is no ground for sinners to conclude that he will not come to judgment, for our Savior has no where fixed and determined the time of it. We can neither be sure when our Lord will come, nor certain when he will not come. Learn, 2. That the true reason why God defers judgment, is to give sinners opportunity for repentance; if this be not complied with, he reserves those who are incorrigibly bad to a more remarkable ruin, condemning them that will not be saved, but obstinately destroy themselves.

William Burkitt, Expository Notes With Practical Observations on the New Testament (Philadelphia: Published by Thomas Wardle, 1835), 2:749-750. [Some spelling modernized; underlining mine.]