Jesus said at Matthew 24:36:
36 “Concerning that day and hour nobody knows, neither the angels of the heavens nor the Son, but only the Father.
Along with that clearly stated truth, Jesus also gave us indications, or a composite sign, of when the end of the world (Greek word kos-mos', literally meaning "system of things") would occur, in Matthew chapter 24, Mark chapter 13, and Luke chapter 21.
Some of the statements there applied to the destruction of Jerusalem in 70 CE, and we have a detailed record of that happening, when 1,1 million people died, and about 10,000 Christians escaped to the mountains of Pella.
But the prophecies in those chapters also applied to a future event, as was indicated by these verses, among many others:
29 “Immediately after the tribulation of those days, the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken. 30 Then the sign of the Son of man will appear in heaven, and all the tribes of the earth will beat themselves in grief, and they will see the Son of man coming on the clouds of heaven with power and great glory. 31 And he will send out his angels with a great trumpet sound, and they will gather his chosen ones together from the four winds, from one extremity of the heavens to their other extremity.
35... For it will come upon all those dwelling on the face of the whole earth.
From the days of the apostles until now, Christians who believed Jesus' words looked for the end of this system of things to happen and hoped that it would happen while they were living on earth.
God has not given it to any human when Jesus will return. We know that he will, because he said he would, and no promise that he has made was ever broken.
Many people accuse specific small religious groups of setting dates, but over the past two thousand years, people from most religions have done so - Catholic, Lutheran, Baptist, Methodist, Adventist, groups who are led by those who say they are prophets, such as Islam and Latter Day Saints, and other groups who have pointed to dates without claiming any special prophetic powers, including Jehovah's Witnesses.
Even the apostles wanted to know if the end would come in their day. It did not, but Jesus did not condemn them, but told them to preach his words to all of the nations. Matthew 28:19-20
Yet Jesus said that we are to "keep on the watch" So the real issue is, "Do we believe him? Are we keeping on the watch?"
And the more pertinent question is: "what side am I on this issue?"
Instead of waiting for God's kingdom to come on earth as it is in heaven, tragically, many churches have left behind that attitude of anticipation, and have tried to make a kind of kingdom of God on earth through the use of terrible violence in order to force their version of Christianity on people, instead of peacefully teaching them as Jesus did.
Augustine's book "City of God", the tortures of the Spanish Inquisition, Thomas Aquinas' "just war" theories, and the slaughter of native American tribes during the European expansion into the New World are examples of promoting this idea. Many of the worst chapters in history featured this terribly wrong view, and it led millions more to reject Christianity..
But Jesus never meant for us to do that. He said:
34 “But pay attention to yourselves that your hearts never become weighed down with overeating and heavy drinking and anxieties of life, and suddenly that day be instantly upon you 35 as a snare. For it will come upon all those dwelling on the face of the whole earth. 36 Keep awake, then, all the time making supplication that you may succeed in escaping all these things that must occur, and in standing before the Son of man.”