[avar]

For the purposes of proving that a document exists no, you really couldn't go back in time, if this were to happen it would be a monumental time for bitcoin, and the old blockchain would be well known & referenced.

So for the purposes of proving that you wrote that document it really doesn't matter if that were to happen.

[ikeboy]

Depends who you're proving it to; if you can isolate them, it still works.