when you fall down the stairs, do you timestamp the moment you tripped, the moment you landed at the bottom, or every moment you hit each and every step on the way down?
In some ways, this is worse, because they hit metaphorical stairs the whole way up as well, and where the "top" is is a matter of what you want to measure.
It's very not-the-circlejerk, but maybe 13 should be "Rome never existed in the first place".
Explanation: Due to Rome's longevity and its wide reach, there are a number of dates that can be used as its fall - some quite, uh, interesting. For bonus points, my date of choice isn't here.
Oh, and for those curious, the most commonly accepted answers are:
476 AD (Fall of the city of Rome and the Western Empire)
1204 AD (Sack of Constantinople and the break of government continuity in the Byzantine Empire)
1453 AD (Siege of Constantinople and conquest of the Byzantine Empire by the Ottoman Turks)
284 AD. The ascension of Diocletian and the shift away from the city of Rome as the center of the Empire. There's still an Empire after that point - but it's only dubiously Roman.
Things got pretty chaotic in the 300s already. The battle of Adrianople or generally the Gothic War would be a classic choice, and you could even cut it off with Constantine or Valerian if you want to emphasis the transition to Christianity and the accompanying cultural shift.
I don't know why I'm guessing, PugJesus is right here...