You can have a third party create a cryptographically signed timestamp for you. For example, secure-timestamp.org will do this. This can only be falsified by getting the timestamping server’s private key or breaking its crypto algorithm. For things more important than Harry Potter predictions, you can have multiple third parties timestamp them for you, in which case falsification requires stealing all of their private keys.
You can have a third party create a cryptographically signed timestamp for you. For example, secure-timestamp.org will do this. This can only be falsified by getting the timestamping server’s private key or breaking its crypto algorithm. For things more important than Harry Potter predictions, you can have multiple third parties timestamp them for you, in which case falsification requires stealing all of their private keys.