Most scholars date the Apocalypse of John around the time of Domitian. There is good reason for this. Both external and internal evidence weighs heavily in its favor. Yet, in recent years, some have attempted to make the book about the fall of Jerusalem in AD 70, and have, therefore, wanted to date the book before that event. Ken Gentry would be an example of this. Hank Hanegraaff would be another popular author. A. T. Robertson attempted this awhile back as well.
The arguments for the early date are wrapped around time references to the event occurring soon and that it is near, the reference to the standing temple in Chapter 11, and the succession of the Caesars in Revelation 13 and 17.
The time references concerning "soon" can be dismissed due to the fact that the word tachu likely means "quickly," not "soon," which is not a reference to when judgment will occur in the context, but how fast it will fall when it does fall. It is the manner of judgment, not the time of it. The time reference translated "near" actually means more along the lines of something that is already here. It is currently in the presence of the object. So I actually think that this indicates something that is not on its way or coming, but a time that has already come, a judge that is already here, "at the door." He's arrived and is now going to make His judgment. This, of course, unfortunately for the early date, however, does not solidify an early date, but just that whatever judgment is taking place is taking place right then and there at the time John is writing.
As for the temple in Chapter 11, this is a clear allusion to Ezekiel's temple vision in Chapter 40. In Ezekiel's vision, the temple he is seeing doesn't exist in Jerusalem. There is simply no temple standing at the time of the vision, which is given to us in the text twice (v. 1: in the 25th year of the exile and in the fourteenth year after the city was taken, i.e., long after the temple was destroyed). But further questions to be asked of the text in Revelation exist, such as why is John measuring the temple instead of the angel, as in Ezekiel's vision, when measuring is done in the Bible by one who has a responsibility over what is measured? Why is Ezekiel's temple in John being viewed as a literal temple when in Ezekiel's vision it is the people of God, not a literal building? Why is John's temple viewed as a literal temple when the temple, throughout the book thus far, refers to the church (e.g., Christ is seen as the One who walks among the golden lampstands, which are the churches within the larger church of the temple in Chapters 2-3)? The people are viewed as a city temple in terms of a New Jerusalem where no literal temple is needed because God is among the people who are identified as the city. In other words, there is no need for a literal temple because the people are the temple. There is actually, then, no literal temple mentioned in the book at all.
Finally, what is perhaps the greatest irony in finding an early date in the succession of Caesars is that both historical fact and the book itself negate this reading. What is done by early date advocates is to start with Julius Caesar as the first Emperor of Rome in an effort to get Nero, i.e., the beast, to be number 6. The problem with this is that Julius is not the first Emperor of Rome. That's a myth based on our literary and legendary culture, perhaps, beginning with Shakespeare. Augustus is the first Roman Emperor, as evidenced, for instance, in things like Trajan's recent coin that was found where the first Emperor is memorialized on the coin, and it's Augustus, not Julius.
But this is actually indicated by the book itself, apart from any knowledge of history as well, as John clearly states that Nero is dead, and therefore, one of the five, not number 6 (17:8 and 10-11). The text clearly states that the beast is not alive and that five of the kings of Rome have already fallen, i.e., are not alive. So let's count backward. If Nero is dead and five of the Emperors of Rome are dead, then Nero is one of the five, and we know that he is the furthest in the line of five, so Nero is five, Claudius is four, Caligula is three, Tiberius is two, and Augustus is one. The return of the beast, who is one of the seven and an eighth, is John's utilization of the Nero redivivus myth. That means that Nero is a fifth Emperor who will manifest himself, likely in terms of his activity, not a literal reincarnation (remember John is using symbols) in the eighth Roman Emperor.
Some may attempt to count the usurpers in the year of the four emperors following Nero, but it is unlikely for anyone to have counted them as legitimate Emperors of Rome against giving that regnal year to Vespasian, and John clearly is not doing that, as Vitellius was hardly Nero come again, as the beast persecutes Christ and Christians when they do not worship him or his image, and Vitellius does none of this. The early date also attempts arbitrarily to count Galba and Otho, but not Vitellius in an attempt to fudge the facts and get to Vespasian as the beast, again, one who destroys Jerusalem, but does not persecute Christians, which is what the beast is doing in the book. This, of course, is selective argumentation, as there is no reason to count two usurpers and not the third. In point of fact, however, these three are not being counted by John.
Instead, Nero is the fifth, Vespasian, the one who would have been proclaimed by Romans as the rightful heir to the throne as opposed to Galba, Otho, and Vitellius, would have been the sixth, Titus the seventh, and Domitian the eighth. Domitian, then, is Nero come again. He demanded everyone to worship through the Imperial cult. He demanded devotion from the greatest to the least. He had himself deified and called "Lord and God" on his coinage. It was after his reign that Pliny the Younger has to try and get reigns on the slaughter that seemed to be reaching the hysteria of the Salem Witch Trials by writing to Trajan about his selective execution of Christians. Domitian is the beast. He is Nero come again in John's argument because that is the persecution going on in the time of John.
He writes the Apocalypse, as apocalyptic books are written, from the standpoint of the past, i.e., during the reign of Vespasian, but he is actually writing during the time of Domitian, much like Daniel does the same with one who is supposedly writing during the exile, but is, in fact, writing during the persecution of Antiochus IV.
Hence, the book itself indicates that it is written during the time of Domitian, not Nero, and the subject matter of the book's background is Domitian's pressure and persecution of the saints who are being pressured under pain of death to worship the beast and his image. Hence, it is both something occurring in the First Century and something that is occurring after AD 70. The macrocosmic language used of the judgment and creation of new heavens and earth is a common literary device of apocalyptic language that brings the world to come into the world now by arguing that the judgment or confirmation of salvation in the present is a piece of what is to come in the future. They are, hence, seen as the event of the future happening in the present on a microcosmic level.
The early date is simply in error, therefore, and the late date evidences that the book is not a prophecy about the destruction of Jerusalem, but is a warning and a comfort to the compromised or persecuted church during a time of great tribulation/trouble/persecution.