The following is an excerpt from The MacArthur New Testament Commentary on 2 Peter 3 .

But do not let this one fact escape your notice, beloved, that with the Lord one day is like a thousand years, and a thousand years like one day. (2 Peter 3:8)

In Psalm 90:4 Moses declared, “For a thousand years in Your sight are like yesterday when it passes by, or as a watch in the night.” Peter’s paraphrase from that psalm encouraged his readers to not let this one fact escape their notice—that God’s perspective on time is much different from humanity’s (cf. Ps. 102:12, 24–27). The amount of earthly time that passes is of no consequence from God’s timeless perspective. A moment is no different from an eon, and eons pass like moments to the eternal God.

What may seem like a long time to believers, like a thousand years, is actually short, like one day, in God’s sight. In context, Peter is contending that, while Christ’s return may seem far off to human beings, it is imminent from God’s perspective. Finite people must not confine an infinite God to their time schedule. The Lord Jesus Christ will return at the exact moment determined by God in eternity past. Those who foolishly demand that God operate according to their time frame ignore that He is the “High and Lofty One who inhabits eternity” (Isa. 57:15, nkjv). Similarly, those who argue that Christ will not return because He has not yet returned demonstrate the height of folly.

Beyond the general sense of a thousand years meaning a long time as opposed to a day meaning a short time, there is also here the specific indication that one thousand years actually lies between the first phase of the day of the Lord at the end of the time of tribulation (Rev. 6:17) and the last phase at the end of the millennial kingdom. At that terminus, the Lord will destroy the universe and create the new heavens and new earth (Rev. 20:1–21:1).