Permanently store this on a public blockchain.
How a timestamp works
We make a fingerprint of your document and have it recorded on Bitcoin through OpenTimestamps, a free public service. You don't sign anything or pay anything.
It usually completes within a few hours. When it does, you'll get a proof file (.ots). Keep it, along with your original document, exactly as it was. Together they show your document existed by that date, and anyone can check it against Bitcoin.
What a timestamp shows, and what it doesn't.
It shows when your exact document existed. It does not show who wrote it, who owns it, or that it is true. It is not an official certification and not a copyright registration. How much weight it carries as evidence depends on where you are and the situation.
Change even one character and the proof no longer matches, so keep your original safe.