Proving you made the statement at a given time is as simple as getting a trusted signed timestamp or inserting it in some blockchain-like ledger, but that's not even close to making yourself accountable for predictive accuracy.

Be sure to publish *all* your predictions so we don't get file-drawered (except on you as a person which we probably can't help).

A common technique is to publish a secure hash of your prediction rather than the text (in case you want to avoid it being self-fulfilled or anti-fulfilled or otherwise traded on) (crypto signed w/ your identity, too).

But if we don't see a stream of plaintext reveals and a means of identifying all such hashes you've published, we might suspect you of planting both positive and negative predictions.

Most people prefer to publish their prediction (+reasoning) clear-text because they want to persuade and they want credit for being smart before the verdict is in.