See for yourself:

3:8–9
The day of the Lord will come as irrevocably as the day of the flood dawned in Noah’s day. From God’s initial announcement of impending judgment to its final execution when he closed the ark, God extended a period of grace to 120 years (Gen. 6:3). Likewise, the day of the Lord will appear at the time God has appointed. True, some people question the coming of this day. Yet that day will come, and then cosmic time, which God has created, will end. God set cosmic time in motion when he made the universe. But when the day of the Lord dawns, chronological time disappears in eternity.

God regards time from a perspective that differs from ours. In the next two verses, Peter teaches the reader to consider both time and patience from God’s point of view.

Relativity
8. But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day.

a. “Dear friends.” Peter now turns his attention to the readers; he exhorts and encourages them by teaching them the significance of time. He addresses them as friends (see vv. 1, 14). The readers are people whom Peter loves and for whom he cares as a pastor. Literally, he addresses them as “beloved.”

b. “But do not forget this one thing.” After they have learned what the destiny of the scoffers will be, the readers are anxious to know what will become of them on the judgment day. Peter has described that day as a day of destruction for the wicked. What will happen to the believer? When will that day come? Although the readers are anxious to multiply their questions concerning the last day, Peter discusses only time itself. In a single sentence, he discloses the relativity of time. He teaches the readers that they should keep one thing in mind: God views time from a perspective that differs from that of man. Introducing this concept, however, Peter tells the recipients of his letter not to “forget this one thing.”

Notice that the verb to forget in verse 8 is the same as in verse 5. There Peter writes that the scoffers deliberately forget pertinent facts concerning creation. Here he exhorts believers not to forget one thing. But what is this “one thing”? Peter sums it up in one sentence:

c. “With the Lord a day is like a thousand years, and a thousand years are like a day.” Peter echoes a verse from a prayer of Moses (Ps. 90:4):
For a thousand years in your sight
are like a day that has just gone by.

Of course, Peter refrains from speculating when the end will come. He knows the word of Jesus on this subject: “No one knows about that day or hour, not even the angels in heaven, nor the Son, but only the Father” (Matt. 24:36). Besides, he knows that God looks at time from the perspective of eternity, and that man, who is conditioned by cosmic time, is unable to comprehend eternity. Peter is not interested in explicating the difference between time and eternity, as Moses does in Psalm 90.27 Rather, he describes time in relation to the last day.

The expression a thousand years occurs in the New Testament only in this verse (v. 8) and in Revelation 20:2–7. Verse 8 provides no information about a literal millennium. In the first two centuries of the Christian era, however, some writers explained this verse in terms of a thousand-year period. These authors used only the words, “With the Lord a day is like a thousand years,” and developed a millennial doctrine. Thus the unknown author of the Epistle of Barnabas writes that “the Lord will make an end of everything in six thousand years, for a day with him means a thousand years.”28 And Irenaeus says, “For the day of the Lord is as a thousand years; and in six days created things were completed: it is evident, therefore, that they will come to an end at the sixth thousand year.”29

These early Christian writers neglect to recite the second part of verse 8, “and a thousand years are like a day.” This cancels out the first part of the verse and, therefore, makes it difficult to develop a millenary theory. Peter is not interested in such theories. He faces the taunts of scoffers who express their doubt about the promise of the Lord’s eventual return (compare v. 4).

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

Because the Christians of the first century expected the imminent return of the Lord and waited patiently, they needed a word of encouragement from Peter.

a. “The Lord is not slow in keeping his promise.” The term Lord in this verse and the next (v. 10) is a synonym for “God.” In other words, Peter refers not to Jesus but rather to God with his use of an Old Testament designation for God. Peter alludes to the Old Testament prophecy of Habakkuk:
For the revelation awaits an appointed time;
it speaks of the end
and will not prove false.
Though it linger, wait for it;
it will certainly come and will not delay. [2:3]

The writer of Hebrews, who assures his readers that God will fulfill the promises that he made to them, quotes this same Old Testament prophecy (see Heb. 10:37). Why does God delay the return of Christ? The cause of the delay stems not from indifference or inattentiveness on the part of God. It lies in God’s grace and mercy toward sinners. He allows them time to repent of their sins. Jesus will return when God’s patience has ended, when the time allotted has expired, and when the last believer has accepted Christ as Savior. “Not human sin, but divine forbearance, which cannot be constrained, determines the delay. It is the sovereign God who graciously grants an interval for repentance.”30 God works out his plan and purpose even though man expresses doubts.

b. “As some understand slowness.” We understand that the “some” in this verse are not the scoffers (v. 3) but believers who have been influenced by these scoffers. Some Christians are unable to explain the delay of Christ’s return and begin to doubt as they listen to the scoffers.31 They need not doubt, because God is in full control.

c. “He is patient with you.” Notice that Peter addresses the readers, not the scoffers, when he writes the pronoun you.32 He indicates that God does not judge his people hastily, but grants them sufficient time to come to repentance (compare I Peter 3:20).

d. “Not wanting anyone to perish.” Peter is not teaching universalism in this sentence. In his epistle, he clearly states that the false teachers and the scoffers are condemned and face destruction (see 2:3; 3:7; Rom. 9:22). Does not God want the false teachers to be saved? Yes, but they disregard God’s patience toward them, they employ their knowledge of Jesus Christ against him, and they willfully reject God’s offer of salvation. They, then, bear full responsibility for their own condemnation.33

d. “[God wants] everyone to come to repentance.” God provides time for man to repent, but repentance is an act that man must perform. Take the case of Esau, who led a godless life and sold his inheritance rights to his brother Jacob. When he wanted to receive the blessing, Esau was rejected. “He could bring about no change of mind, though he sought the blessing with tears” (Heb. 12:17; also compare Rom. 2:4). Likewise the scoffers in Peter’s day refuse to come to repentance, even though God is granting them a period of grace.

Doctrinal Considerations in 3:8–9
“So wonderful is [God’s] love towards mankind, that he would have them all to be saved, and is of his own self prepared to bestow salvation on the lost.”34 So writes John Calvin on verse 9 and thus touches the doctrine of God’s mercy toward sinful man. Here are two examples of this divine love; one is from the Old Testament, the second from the New Testament.

First, God showed his love to Cain when he asked, “If you do what is right, will you not be accepted?” (Gen. 4:7). Yet Cain, filled with anger and jealousy, murdered Abel (v. 8). When God continued to speak to Cain, he did not meet a repentant sinner but a selfish individual who sought protection from an avenger. “Cain went out from the Lord’s presence” (v. 16), although God demonstrated mercy by shielding him (v. 15). Cain belonged to the evil one, says the apostle John (I John 3:12). That is, he rejected God’s grace and mercy and willfully departed from God.

Second, before Jesus appointed the twelve disciples, he spent a whole night in prayer (Luke 6:12). He called Judas Iscariot to the circle of his immediate followers. After a period of instruction, Jesus commissioned the twelve disciples to preach the gospel, to heal the sick, to raise the dead, to cleanse the lepers, and to drive out demons (Matt. 10:7–8). In his love, Jesus commissioned Judas, too. Even at the last Passover celebration in the upper room, Jesus visibly indicated to Judas that he knew of the betrayal (John 13:26). Yet Judas delivered his Master to the chief priests. Granted that Judas was filled with remorse, he never repented (Matt. 27:3). He never returned to Jesus, but instead committed suicide.

When Paul writes that God “wants all men to be saved and to come to a knowledge of the truth” (I Tim. 2:4; also see Ezek. 18:23, 32), he does not mean that all men are indeed saved. Although God desires the redemption of the entire race, he does not decree universal salvation. Therefore, in respect to the verb want or wish theologians distinguish between God’s desire and God’s decree.35

God extends his mercy to sinful man. However, when man repudiates God’s grace, divine condemnation hangs over him (II Peter 2:3) and he faces the inevitable day of judgment (3:7).

Kistemaker, Simon J. ; Hendriksen, William: New Testament Commentary : Exposition of the Epistles of Peter and the Epistle of Jude. Grand Rapids : Baker Book House, 1953-2001 (New Testament Commentary 16), S. 331