2 Peter 3:8

6 through which the world of that time perished in the flood.

7 And by that same word, the present heavens and earth are reserved for fire, being kept for the day of judgment and destruction of ungodly men.

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.

9 The Lord is not slow in keeping His promise as some understand slowness, but is patient with you, not wanting anyone to perish but everyone to come to repentance.

10 But the Day of the Lord will come like a thief. The heavens will disappear with a roar, the elements will be destroyed by fire, and the earth and its works will be laid bare.

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