With this phrase Peter alludes to Psalm 90:4. Psalm 90 is a song which speaks about the judgment of God that comes upon humanity (Psalm 90:3, Psalm 90:5–11) as well as the temporary nature of human life in contrast with the eternal nature of God (Psalm 90:4).1 Peter uses this psalm to compare our human perception of time with that of God. The criteria for what is quick and what is slow is different for us as humans than what it is for God. God is not limited by a human life span; he surveys the whole course of history.2 Thus, Peter is not saying that one day for God is the same as a 1,000 years for humanity.
8 But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day.