External sources, Origin, Josephus, etc. have indicated that John was exiled to Patmos by Domitian (sp?).
That's a pretty good time indicator.
Exactly. All of the attestation for this comes from patristic sources, not from Scripture itself. That's my point; it flunks the sola scriptura test quite decisively.
Josephus had nothing to say about it; neither does Origen. The original testimony seems belong to Irenaeus, who wrote:
We will not, however, incur the risk of pronouncing positively as to the name of Antichrist; for if it were necessary that his name should be distinctly revealed in this present time, it would have been announced by him who beheld the apocalyptic vision. For that was seen no very long time since, but almost in our day, towards the end of Domitian's reign.
People writing after him, like Eusebius or Jerome, may be passing on an unreliable oral tradition based on a particular interpretation of Irenaeus' words.
Irenaeus' own statement is ambiguous, and even more ambiguous in the Greek. It could mean that "the apocalyptic vision" was seen towards the end of Domitian's reign, or it could mean that John himself was seen, that is, was still alive, as recently as the end of Domitian's reign.
The text itself argues for a date before AD 70. The opening verse says that it describes things which must soon take place, and the only reasonable candidate for those "things" which could have happened "soon" would be the fall of Jerusalem, which is a "type" or foreshadowing for the end of the world.