Is there a cryptographic mechanism by which it is possible to sign a document with a date, such that it is not possible to forge that same signature at a later date? Maybe some sort of server that publishes daily cryptographic keys (but how can you trust them? ;-).
For the inevitable prodding pragmatists, I'm not trying to accomplish some task. I'm just curious what the solution space is like.
This is called Timestamping (TSP protocol, RFC 3161). Different digital signature standards (PDF and XML signatures, CAdES, PAdES, XAdES) include support for advanced timestamping based on TSP.
MS Authenticode also includes timestamping, but uses different (incompatible and less secure) mechanism for it.
TSP alone (without signature protocols) is not used a lot, but in conjunction with signature standards it becomes very handy.