Historically poorman's copyright evidence has been ruled as inadmissible; it's too easy to forge.

Moreover it doesn't prove that the work is yours; all is might do is possibly add information about when the work in that form existed. It doesn't rule out that it was a prior work or a derivative work.

It's stupid.

Also, with respect to "speed writing" 98%, or possibly higher, of all manuscripts are unpublishable (and often, unreadable).

There's little or no value in an uncontracted and unpublished ms.

That said, if you have your original materials, notes, drafts, crit group responses, beta reader notes, etc. you can prove in court that you are the creator and you own all the rights, not the putative thief.

And I think we're done here.