Very good question. I have also occasionally wondered about this and haven't come up with a satisfying solution. I guess it boils down to 1) reference in your m

Very good question. I have also occasionally wondered about this and haven't come up with a satisfying solution. I guess it boils down to 1) reference in your message something that didn't exist before your point-in-time 2) let a trusted third party publish the hash of the message you compiled. A third party seems necessary because time isn't a technical concept, but time can be described as a sequence of events - so some trusted event needs to reference your event/document(its hash or equivalent). As a comment here mention, blockchain could be used as a solution. Really interested to see other solutions in this thread.