Most of the books of the New Testament can be dated with reasonable certainty. We know when Peter and Paul lived and died, for example, and their lives can be cross-checked with events in the book of Acts, which in turn can be aligned with Roman history to a large degree. The case of John and Revelation is harder, and the interpretive stakes are higher.
The key question for the date of Revelation is whether it was written before or after AD 70, the year the Jewish temple was destroyed. If Revelation was written before that date, much of the content of the book could be interpreted as leading to that cataclysmic event. That would mean most (some would say all) of the prophecies in the book have already been fulfilled. If it was written after AD 70, then the book really can’t be viewed that way—the prophecies would be still awaiting fulfillment. That’s a huge interpretive gap.
There is no clear reference to the temple being destroyed within the book itself. That suggests that the event had not happened—which would favor a pre–AD 70 date for the book. The other view—that the book was written after AD 70—objects that this is an argument from silence. The debate opens with those basic differences. Then Revelation 11:1–2 comes into play:
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.”
Do John’s words indicate that Jerusalem and its temple were literally still standing and under attack? If that’s the case, then the city was destroyed in three and one-half years, and the book was written before AD 70. And that, in turn, is a serious reason to think it is not pointing to a future beyond our time.
Ironically, the view that sees the prophecies as “literally” future oriented must still interpret this passage symbolically—that Revelation 11:1–2 wasn’t about the actual temple a few years before AD 70. That view makes its argument by appealing to other items in the book. For example, it is argued that “Babylon” in the later chapters of the book actually refers to Rome (see Rev 14:8; 16:19; 17:5; 18:2, 10, 21). The basis for this suggestion is that other books in the ancient world that are datable to the late first century AD use the term “Babylon” as code for Rome. If Revelation is using that code, it suggests that the book was written after AD 70. Irenaeus, an important Christian writer from the generation that followed the apostles, wrote that the book was written “toward the end of Domitian’s reign” in the late first century AD.
We can’t know for sure who’s right. That’s par for the course for Revelation.