While I have long held to the necessarily-early dating of the writing of the Revelation, I am indebted for the reasoning below to Rev. Kenneth Gentry's book "Postmillennialism Made Easy."
The general view among evangelicals is that the Apostle John wrote the Revelation in the early 90's AD, during the persecution under Emperor Diocletian. Liberal theologians time it much later, and deny the apostolic authorship. However, I don't take that view into consideration. Gentry, and many in the preterist and postmillenialist crowd, time it about 25 years earlier, during the reign of Emperor Nero. I find their reasoning, based on the internal evidence, to be compelling.
First, John writes with a view of a standing Temple. Revelation 11:1-2 reads, "Then I was given a measuring rod like a staff, and I was told, 'Rise and measure the temple of God and the altar and those who worship there, but do not measure the court outside the temple; leave that out, for it is given over to the nations, and they will trample the holy city for forty-two months.'" Since the Romans destroyed the Temple in 70AD, as Jesus Himself prophesied in Matthew 24:2, then the Revelation must have been written before that date. Notice especially the parallel wording used by John here and the words of Jesus in Luke 21:24.
Now go down to Revelation 17:1-6, the vision of a seven-headed beast. Verse 9 then interprets the vision: the seven heads represent seven mountains, surely a reference to the renowned seven hills of Rome. Then verse 10 adds that the heads also represent seven kings. The presumption is strong that these would then be kings of Rome. John specifies that there were five past kings in the chain, one reigning currently, with his successor doomed to reign only briefly. The first seven emperors of Rome were Julius Caesar (49-44BC), Augustus (31BC-14AD, see Luke 2:1), Tiberius (14-37AD, see Luke 3:1), Gaius (37-41AD), Claudius (41-54AD, see Acts 11:28 and Acts 18:2), Nero (54-68AD. see Acts 28:19), and Galba (June, 68-January, 69AD). So, sixth Emperor Nero's death in 68 (followed by the six-month reign of Galba), requires that the Revelation have been written no later that his death on June 6, 68AD, an historically-objective circumstance not subject to anyone's theological presuppositions.
Where this leads me is to the expectation that John was describing the upcoming destruction of Jerusalem, the Temple, and the sacrificial system, in the Revelation. Not exhaustively, since I am conscious obviously that the resurrection and Second Coming are yet to be fulfilled. However, this is logically the predominant focus of the book. Not the European Union or Social Security numbers or the myriad of other passing fantasies that arise daily in the "prophetic" crowd, most-recently from Harold Camping.