Mat 24:35 Heaven and earth shall pass away, but my words shall not pass away.
Mat 24:36 But of that day and hour knoweth no [man], no, not the angels of heaven, but my Father only.
Note the change from verse 34 to verse 36. In verses 4-34 Jesus is giving a very detailed description of all the events that would precede the great tribulation. He a speaks of persecution of the saints, war and rumors of wars, famine, pestilence, the abomination of desolation which Luke tells us is related to Jerusalem being surrounded by armies (Luke 21:20). Jesus warns His disciples listening to His words, those living in Judea, to flee to the mountains. This detailed description from Jesus fits the events that fell upon Jerusalem in AD70 when the armies of Rome sacked the city and destroyed the ancient temple, so that not one stone shall be left here upon another. All these things came upon this generation (v. 34).
In verses 36 and following we see a different view of things. There are no warnings about signs and events. We are told that things are going on pretty much as normal, marring and being given in marriage, etc (v. 38). The suddenness of the event is compared to the sudden flood that came upon the generation of Noah in his day. There was no warning by way of signs until it started to rain and Noah and his family entered the ark.
I believe vv 4-34 is speaking of the events of AD70, very detailed so that the disciples had tiem to flee from a doomed city, and that vv. 36ff is speaking of the Second Coming that will be without warning so that no one knows the day or hour.
there is a way to know. Follow this series and you will see!