TRADITION VS SCHOLARSHIP
The answer
Nero. Counting from Julius Caesar, Nero is the sixth ruler of Rome, the "one living." Count from Augustus, the first emperor, and the sixth is someone else, which is one reason the verse does not settle the date.
Why it’s interesting
The verse may also date only that one vision, not the whole book. One standard introduction weighs clues like this and concludes that parts of Revelation were written in the 60s and the whole was finished around 95 AD.
Genuinely debated
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Not just the stories, but the history, authorship, and scholarship behind it.