So why is this not a problem? Let's look at the scriptures and deal with three accounts being pointed towards.
28 “Now learn this lesson from the fig tree: As soon as its twigs get tender and its leaves come out, you know that summer is near. 29 Even so, when you see these things happening, you know that it[d] is near, right at the door. 30 Truly I tell you, this generation will certainly not pass away until all these things have happened. 31 Heaven and earth will pass away, but my words will never pass away. - Mark 13:28-31
32 “Now learn this lesson from the fig tree: As soon as its twigs get tender and its leaves come out, you know that summer is near. 33 Even so, when you see all these things, you know that it[e] is near, right at the door. 34 Truly I tell you, this generation will certainly not pass away until all these things have happened. 35 Heaven and earth will pass away, but my words will never pass away. - Matthew 24:32-34
29 He told them this parable: “Look at the fig tree and all the trees. 30 When they sprout leaves, you can see for yourselves and know that summer is near. 31 Even so, when you see these things happening, you know that the kingdom of God is near.
32 “Truly I tell you, this generation will certainly not pass away until all these things have happened. 33 Heaven and earth will pass away, but my words will never pass away. Luke 21:29-33
Many skeptics say we should try to read the Bible horizontally. Let's do that. The question is what does "generation" mean? The events Jesus is referring to can't mean his second advent given the verses that comes from directly after their proof texting.
32 “But about that day or hour no one knows, not even the angels in heaven, nor the Son, but only the Father. 33 Be on guard! Be alert! You do not know when that time will come. 34 It’s like a man going away: He leaves his house and puts his servants in charge, each with their assigned task, and tells the one at the door to keep watch.
35 “Therefore keep watch because you do not know when the owner of the house will come back—whether in the evening, or at midnight, or when the rooster crows, or at dawn. 36 If he comes suddenly, do not let him find you sleeping. 37 What I say to you, I say to everyone: ‘Watch!’” - Mark 13:32-37
36 “But about that day or hour no one knows, not even the angels in heaven, nor the Son,[f] but only the Father. 37 As it was in the days of Noah, so it will be at the coming of the Son of Man. 38 For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark; 39 and they knew nothing about what would happen until the flood came and took them all away. That is how it will be at the coming of the Son of Man. 40 Two men will be in the field; one will be taken and the other left. 41 Two women will be grinding with a hand mill; one will be taken and the other left.
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. - Matthew 24:36-44
34 “Be careful, or your hearts will be weighed down with carousing, drunkenness and the anxieties of life, and that day will close on you suddenly like a trap. 35 For it will come on all those who live on the face of the whole earth. 36 Be always on the watch, and pray that you may be able to escape all that is about to happen, and that you may be able to stand before the Son of Man.” - Luke 21:34-36
So why would Jesus in one breath say "I'll be back in your lifetime." And then in the next breath say "no one knows the day or hour" and to live as though it could be at any moment." Does that make sense? That is a major contradiction and there is no reason to add it. Even if you think that the Gospels are not inspired or written according to eyewitness testimony, you have to admit that it makes no sense for a single author to conflict himself so strikingly. It makes more sense to agree that Jesus was not saying the Judgment day was promised before Jesus' hearers died. Many scholars today actually look at these passages as referring to more than just Jesus' second coming but refers to the destruction of the 2nd Temple and Jerusalem in 70 AD. Some even think that the entire passage only refers to 70 AD. I'm not willing to go that far.
As for what Jesus meant by "generation", the word was translated from the Greek word "Genea". Genea can refer to the group of people living at the same time. However "race" or "that which has been begotten, men of the same stock, a family". Jesus clearly was not saying he would return before everyone who was listening him died. He did promise that they would see their nation destroyed before that group who was listening to him all died out.
Recall what the disciples were asking Jesus. They asked him:
3 As Jesus was sitting on the Mount of Olives, the disciples came to him privately. “Tell us,” they said, “when will this happen, and what will be the sign of your coming and of the end of the age?” - Matthew 24:3
Was Jesus wrong? No he wasn't. He answered their question. I don't think it can be shown that those who heard Jesus heard what Loftus reads into Jesus' words now. I do think the Bible bears out that they did think Jesus was going to return in their lifetimes. Jesus told them to live as if He was going to return in their lifetimes. Just as we should today. However, Jesus did not set at a date. Neither should we.
Debunking Christianity: Harold Camping Step Aside, Jesus Was Wrong And Should Be Ignored Too