If you combine public key crypto with parent's suggestion, you get a claim of the kind: "I can prove the timestamp is correct, and you'll have to trust me that

If you combine public key crypto with parent's suggestion, you get a claim of the kind: "I can prove the timestamp is correct, and you'll have to trust me that the content is what I claim it to be", which is stronger than just doing public-key crypto. It's not a solution of the full problem, but it's something that should cover most common use cases, unless you have pretty weird requirements.