[GrinningFool]

Mail it to yourself.. You'll get a postmark with a date/time it was processed. Don't open the envelope once it arrives, and now you've got proof of when you mailed it.

[lkbm]

I remember people suggesting this as a poor man's "copyright" on an email list back in the day.

Others quickly pointed out that you can mail yourself an unsealed envelope and put something in it after it arrives.

[GrinningFool]

Pardon me while I apply palm to forehead with middling force.

[johnfactorial]

This is the little detail my dumb joke missed.