2 Peter 3:8 likens a day to a thousand years in the sight of God. A similar expression is found in Psalm 90:4:
For a thousand years in your sight are like a day that has just gone by, or like a watch in the night.
Neither passage is saying a year is a thousand years, only that when viewed in the light of eternity, it could be likened to a thousand years, or even “a watch in the night” which is but a few hours. The Jews divided the night into three watches (Judges 7:19; 1 Samuel 11:11) but the Romans had four night watches, each lasting three hours (Matthew 14:25).
There are other biblical examples that illustrate how time can be viewed. “The day of the Lord” (1 Thessalonians 5) is not a literal 24-hour day. It is an unspecified period of time.
We are still in the seventh day after God finished his work of creation, His day of rest (Genesis 2:1-3).
Jesus, quoting from Isaiah 61:1, spoke of “the year of the Lord’s favour” (Luke 4:19). That “year” is still with us.
God, as the creator of time, does not view time as mortal humans view time. Our allocated “three score years and ten” are but a fleeting moment. God stands above time, and He waits patiently for humans to repent and turn to Him (2 Peter 3:9).
There is, however, a danger in taking the expression “a day is like a thousand years” and applying it literally to biblical chronology. Some denominations have used this as a means of measuring time in order to predict the tribulation period or the second coming of Christ Jesus. No man knows the day or the hour when He will return to bring judgment on God’s enemies. The day of the Lord will come like a thief in the night (2 Peter 3:10) when people least expect it.
“A day is like a thousand years” is a figure of speech which makes a description more emphatic or vivid. The expression simply brings home the fact that God, the Alpha and the Omega, the beginning and the end, sees time through the eyes of eternity, and not through the eyes of human experience.