This plugin allows you to cryptographically timestamp your posts and pages using https://opentimestamps.org That means you can use the blockchain to prove that you did write a specific thing at a specific time.
To enable it, just install the plugin, it will timestamp each and every post and page if it's not timestamped yet.
If the contents of a post or page change, it will be timestamped again.
This plugin makes no effort to preserve old content or old timestamps. If you modify your content, it's you are the one that has to keep a copy of your content and the matching timestamp file. Keep in mind that the timestamp only has a hash of the file, not the contents, so it's not enough to prove anything you also need to keep a copy of the contents you timestamped.
I suggest you put everything on git, but you do what you want.