[senstax]

Many are mentioning digital timestamps. There's a pretty universal physical one, too -- a postmark. So enclose your message and a newspaper front page in an envelope addressed to yourself and sealed in a trustable way, and drop it in the mail. But then opening it can only happen once and has to be witnessed.

[rthille]

You just mail the unsealed envelope, add the content later and you’ve forged the time.

[oneplane]

You can just buy a postmarking machine... Get any postmark you like.