The text says ‘one day is like [or as] a thousand years’—the word ‘like’ (or ‘as’) teach that Lord (κυρίῳ) is outside of time as we know it.
Which means for the heavenly beings there is no distinction between a thousand years and a day, therefore the time is just an illlusion.
Some people teach that the days of Genesis might be 1000 years.
In any case, the meaning of ‘day’ in Genesis 1 is defined by the context there—the Hebrew word for day, yôm יום , is used with the words
‘evening’ and ‘morning’, and the days are numbered (first day, second
day, etc.). Whenever yôm is used in such a context, it is always an
ordinary day, never a long period of time.
But they forget the fact that the passage is actually contrasting a short and long period can be shown by the fact that Peter is quoting Psalm 90:4 (Peter’s statement ‘do not forget’ implies that his readers were expected to recall something, and this passage has this very teaching). This reads:
"A thousand years in your sight are like a day that has just gone by,
or like a watch in the night." (New International Version)
For further info, please read below commentaries:
God is eternal: his thought is not, like ours, subject to the law of
time; and even we can understand that one day, as the day of the
Saviour's death, may have far more of intense action compressed into
it, and far more influence upon the spiritual destiny of mankind, than
any period of a thousand years.
"the six days of the creation (they say (e)) is a sign or intimation
of these things: on the sixth day man was created; and on the seventh
his work was finished; so the kings of the nations of the world
(continue) five millenniums, answering to the five days, in which were
created the fowls, and the creeping things of the waters, and other
things; and the enjoyment of their kingdom is a little in the sixth,
answerable to the creation of the beasts, and living creatures created
at this time in the beginning of it; and the kingdom of the house of
David is in the sixth millennium, answerable to the creation of man,
who knew his Creator, and ruled over them all; and in the end of that
millennium will be the day of judgment, answerable to man, who was
judged in the end of it; and the seventh is the sabbath, and it is the
beginning of the world to come."
one day … thousand years—(Ps 90:4): Moses there says, Thy eternity,
knowing no distinction between a thousand years and a day, is the
refuge of us creatures of a day. Peter views God's eternity in
relation to the last day: that day seems to us, short-lived beings,
long in coming, but with the Lord the interval is irrespective of the
idea of long or short. His eternity exceeds all measures of time: to
His divine knowledge all future things are present: His power requires
not long delays for the performance of His work: His long-suffering
excludes all impatient expectation and eager haste, such as we men
feel. He is equally blessed in one day and in a thousand years. He can
do the work of a thousand years in one day: so in 2Pe 3:9 it is said,
"He is not slack," that is, "slow": He has always the power to fulfil
thousand years as one day—No delay which occurs is long to God: as to
a man of countless riches, a thousand guineas are as a single penny.
God's oeonologe (eternal-ages measurer) differs wholly from man's
horologe (hour-glass). His gnomon (dial-pointer) shows all the hours
at once in the greatest activity and in perfect repose. To Him the
hours pass away, neither more slowly, nor more quickly, than befits
His economy. There is nothing to make Him need either to hasten or
delay the end. The words, "with the Lord" (Ps 90:4, "In Thy sight"),
silence all man's objections on the ground of his incapability of
understanding this [Bengel].
Source: Parallel Commentaries at Bible Hub