Studies have shown it was written about 67-69 AD. That it was before the destruction of Jerusalem (70AD) is clear since in the book the temple is still standing, and one of the major portions of the Revelation is devoted to the description of the Roman invasion of Palestine that ended in the destruction of the temple and the city. During this horrible event over 3 million people were killed. No wonder Jesus spoke of it as a time that was not like any other before or after!
At the outset, the book tells us that the “time is at hand.” The same words were repeated in the final chapter. The book was not to be sealed as was the book of Daniel (in direct contrast to what was said there in the last chapter of his prophecy). Daniel’s material, in part, was for the far off future. So it was sealed until “the time of the end” (of the Old Testament period). Because, in contrast, Revelation spoke of times that those who first read it would live to see, John was instructed not to seal his book—and, note the reason given: “for the time is at hand!”
The list of Roman rulers, culminating in Nero Caesar (666), was contemporary. The angel tells the reader that the woman was sitting on the city of seven hills (everyone knew Rome that way). Moreover the angel calls it the great city that is reigning over the rest of the earth. That the book is contemporary, and (except for the 20th and 21st chapters) not still future, is clear from the internal evidence.
If you want to learn more, read my book, The Time is at Hand.