[hybridsole]

Computationally difficult is a bit of an understatement. It is not possible to alter the bitcoin blockchain once a block is added and confirmed. With limitless resources, you could perform a withholding attack to modify the most recent half-hour to one hour's worth of transactions, but that would take tens of millions of dollars in hardware and only be temporarily successful.

[ikeboy]

You could go back in time indefinitely with unlimited resources. Would require a big reorg, but still possible.

[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.