But Jesus is coming at the end of the age. The following passage indicates that heaven and earth will pass away at His second coming.
2 Peter 3:3 Above all, you must understand that in the last days scoffers will come, scoffing and following their own evil desires. 4 They will say, “Where is this ‘coming’ he promised? Ever since our ancestors died, everything goes on as it has since the beginning of creation.” 5 But they deliberately forget that long ago by God’s word the heavens came into being and the earth was formed out of water and by water. 6 By these waters also the world of that time was deluged and destroyed. 7 By the same word the present heavens and earth are reserved for fire, being kept for the day of judgment and destruction of the ungodly. 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. 9 The Lord is not slow in keeping his promise, as some understand slowness. Instead he 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 everything done in it will be laid bare. 11 Since everything will be destroyed in this way, what kind of people ought you to be? You ought to live holy and godly lives 12 as you look forward to the day of God and speed its coming. That day will bring about the destruction of the heavens by fire, and the elements will melt in the heat. 13 But in keeping with his promise we are looking forward to a new heaven and a new earth, where righteousness dwells.
The context of the entire passage is established in the first 2 verses. This is all about the promise of Christ's second coming. In relation to His second coming, Peter indicates in verse 7 that the heavens and earth are reserved for fire just as the world was destroyed by water in Noah's day. He made the same comparison to what will happen on the day Christ returns to what happened in Noah's day that Jesus did in Matthew 24:37-39.
Then in verse 9 Peter indicates that the Lord is not slow in keeping the promise of His second coming. So, the subject is still the second coming of Christ at that point. And then Peter proceeds to describe what he had mentioned earlier in verse 7 in more detail in verses 10-12. Clearly, what is described there is in relation to the second coming of Christ and Him coming like a thief in the night.
And, finally, Peter indicates that what will result from Him keeping the promise of His second coming is the new heaven and new earth where righteousness dwells.
So, coming back around to Matthew 24:36, we should understand that what He was saying there is that no one knows the day or hour of His second coming which will be when heaven and earth pass away.
We also know that He was referring to His second coming in Matthew 24:36 by simply looking at other verses like Matthew 24:42-44 and Matthew 25:13.
Matthew 24:42 “Therefore keep watch, because you do not know on what day your Lord will come. 43 But understand this: If the owner of the house had known at what time of night the thief was coming, he would have kept watch and would not have let his house be broken into. 44 So you also must be ready, because the Son of Man will come at an hour when you do not expect him.
Jesus Himself said "you do not know on what day your Lord will come". With that in mind, how can you think that He was not referring to His coming in Matthew 24:36 that He had just talked about in Matthew 24:29-31?
Matthew 25:13 “Therefore keep watch, because you do not know the day or the hour.".
This is what Jesus said right after describing His second coming in a parable, so it's very obvious that He was saying you do not know the day or hour that I will come. But, what many don't catch is that He was saying in Matthew 24:35-36 that heaven and earth will pass away when He comes which lines up with what Peter taught.