So you are saying that 2 Peter 3:8 is or is not a prophesy of scripture since it is written in Psalms 90:4
For a thousand years in thy sight are but as yesterday when it is past, and as a watch in the night.
So in 2 Peter 3:8, the writer who is Peter, is prophesying his interpretation of the scripture written in
Psalms 90:4, which is evident by the context. The writer isn't saying that a day with with the LORD is a thousands years, which would be the case if the words were written by the LORD in either scripture.
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.
But Psalms 90 is referring unto man learning to number his days, in specific, Genesis 6:3 which sets the number of days of man in the flesh upon earth. However, the son of David also wrote in Ecclesiastes the following ;
Yea, though he live a thousand years twice told, yet hath he seen no good: do not all go to one place?
But in conclusion, as written in Jer 23:28, "The prophet that hath a dream, let him tell a dream; and he that hath my word, let him speak my word faithfully. What is the chaff to the wheat? saith the LORD."