Look for RFC3161, its a standardized timestamping service over the network. You send arbitrarily binary content (subject to size limitations of course) via HTTP

Look for RFC3161, its a standardized timestamping service over the network. You send arbitrarily binary content (subject to size limitations of course) via HTTP POST to an endpoint of the provider, and the resulting HTTP response is a signed data structure containing a cryptgraphic hash of the response together with the timestamp. When public/private crypto is used by the provider, all you need to verify the signed response for authenticity is the public key of the provider.