[jamieweb]

I looked into this a few years ago[1]. The solution I came to involves putting the hash of the most recent Bitcoin block in your document, and then signing the hash of your document into the Bitcoin blockchain.

This cryptographically proves that the document was created/published within those two time bounds.

This of course doesn't help in all cases, i.e. you could edit an old document to make it look newer, but I'm not aware of any way to properly solve that particular problem without a trusted third-party.

[1] https://www.jamieweb.net/blog/proof-of-timestamp/