The early church fathers, Eusebius, Irenaeus, and Clement of Alexandria tell us that John was exiled to Patmos in the reign of Domitian, who only became emperor AFTER the fall of Jerusalem:
http://emp.byui.edu/SatterfieldB/Rel212/Eusebius/Eusebius%20on%20John.html
I’ve never found any sources of such early date which disagree with this assessment. The only sources that claim an earlier date come many centuries later, after the false doctrine of preterism was invented.
I am not a preterist but I think that the “little book” that John ate was his Gospel. That explains why it is different from the others.