For legal purposes, it is often useful to have a document notarized to prove it was created no later than that date. Digital documents could benefit from the same treatment.
A user first calculates a hash of their document, and uploads it to a server. The server then appends a timestamp, and clearsigns
both with its private key. The result is presented to the user for safe keeping.
To verify the timestamp, a second user calculates the hash from the same document, and compares it against the clearsigned stamp. If the hashes match, and the cryptographic signature matches according to the server's public key, the user can be reasonably certain the document was created at or before the time of signing.
It would be even more beneficial to extend this service for full legal use. To do so, it would need to be administrated by a governing body such as the NIST. The volume of data is so low, the hosting costs would be minimal, but of course there must be a strong focus on security.