Preterists teach that the Book of Revelation is primarily a prophecy about the Roman war against the Jews in Israel that began in A.D. 67 and ended with the destruction of the Temple in A.D. 70. In order for Revelation to be a prediction of the future (Rev. 1:1, 3, 11, 19; 22:6-10, 16, 18-20) and if it was fulfilled by August A.D. 70, then it had to have been written by A.D. 65 or 66 for the preterist interpretation to even be a possibility. Preterist Ken Gentry has noted this major weakness when he said of fellow early date advocate David Chilton, “if it could be demonstrated that Revelation were written 25 years after the Fall of Jerusalem, Chilton's entire labor would go up in smoke.”1 Actually, all one would have to do is to show that Revelation was written any time after the destruction of Jerusalem.
The futurists interpretation is not dependant upon the date of Revelation since it does not matter when these events take place since they are still future to our own time. However, the date of Revelation is essential to the preterist position and explains why they are so focused upon defending an early date. There are two lines of evidence: external (evidence from outside the Revelation) and internal (evidence from inside the Revelation).
Ice, Thomas D., "The Date of the Book of Revelation" (2009). Article Archives. 75.