"But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day" (2 Pet. 3:8).
Milton Terry refuted this seemingly plausible but spurious theory that this means, it is said, that "God's arithmetic is different from ours," so that when Scripture uses terms like "near" and "shortly" (e.g., Rev. 1:1-3) or "at hand" (e.g., James 5:5-7), it doesn't intend to give the impression of soon-approaching events, but of events possibly thousands of years in the future…
The language is a poetical citation from Psalm 90:4, and is adduced to show that the lapse of time does not invalidate the promises of God....But this is very different from saying that when the everlasting God promises something shortly, and declares that it is close at hand, He may mean that it is a thousand years in the future. Whatever He has promised indefinitely He may take a thousand years or more to fulfill; but what He affirms to be at the door let no man declare to be far away. [Milton Terry, Biblical Hermeneutics]
J. Stuart Russell wrote with biting disdain:
Few passages have suffered more from misconstruction than this, which has been made to speak a language inconsistent with its obvious intention, and even incompatible with a strict regard to veracity. There is probably an allusion here to the words of the Psalmist, in which he contrasts the brevity of human life with the eternity of the divine existence....But surely it would be the height of absurdity to regard this sublime poetic image as a calculus for the divine measurement of time, or as giving us warrant for wholly disregarding definitions of time in the predictions and promises of God.
Yet it is not unusual to quote these words as an argument or excuse for the total disregard for the element of time in the prophetic writings. Even in cases where a certain time is specified in the prediction, or where such limitations as 'shortly,' or 'speedily,' or 'at hand' are expressed, the passage before us is appealed to in justification of an arbitrary treatment of such notes of time, so that soon may mean late, and near may mean distant, and short may mean long, and vice versa....
It is surely unnecessary to repudiate in the strongest manner such a non-natural method of interpreting the language of Scripture. It is worse than ungrammatical and unreasonable, it is immoral. It is to suggest that God has two weights and measures in His dealings with men, and that in His mode of reckoning there is ambiguity and variableness which will make it impossible to tell 'What manner of time the Spirit of Christ in the prophets may signify' [cf. 1 Pet. 1:11]... The Scriptures themselves, however, give no countenance to such a method of interpretation. Faithfulness is one of the attributes most frequently ascribed to the 'covenant-keeping God,' and the divine faithfulness is that which the apostle in this very passage affirms....The apostle does not say that when the Lord promises a thing for today He may not fullfil His promise for a thousand years: that would be slackness; that would be a breach of promise. He does not say that because God is infinite and everlasting, therefore He reckons with a different arithmetic from ours, or speaks to us in double sense, or uses two different weights and measures in His dealings with mankind. The very reverse is the truth....
It is evident that the object of the apostle in this passage is to give his readers the strongest assurance that the impending catastrophe of the last days was on the very eve of fulfillment. The veracity and faithfulness of God were the guarantees of the punctual performance of the promise. To have intimated that time was a variable quantity in the promise of God would have been to stultify and neutralize his own teaching, which was that 'the Lord is not slack concerning His promise.' [J. Stuart Russell, The Parousia]
[David Chilton, Looking for New Heavens and a New Earth]