2 Peter 3:8

6 Whereby the world that then was, being overflowed with water, perished:

7 But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of ungodly men.

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.

9 The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.

10 But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.

Do read the surrounding verses too, because the context often says more than a single verse.

Compare

BSB
Beloved, do not let this one thing escape your notice: With the Lord a day is like a thousand years, and a thousand years are like a day.
KJV
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.
WEB
But don’t forget this one thing, beloved, that one day is with the Lord as a thousand years, and a thousand years as one day.
ASV
But forget not this one thing, beloved, that one day is with the Lord as a thousand years, and a thousand years as one day.
DARBY
But let not this one thing be hidden from you, beloved, that one day with [the] Lord [is] as a thousand years, and a thousand years as one day.
BLB
But let this one [thing] not be hidden from you⁺, beloved, that with the Lord one day [is] as a thousand years, and a thousand years as one day.
ÅB
Men ett må dere ikke overse, dere elskede: én dag er i Herrens øyne som tusen år, og tusen år som én dag.
OTB
Men glem ikke denne ene sannhet, elskede: for Herren er en enkelt dag som tusen år, og tusen år som en enkelt dag.
B1930
Men dette ene må I ikke være blinde for, I elskede, at én dag er i Herrens øine som tusen år, og tusen år som én dag.

Study notes (Tyndale)

Original text and word study

Cross references