I feel like the best you can do is either to publish a cryptographically secure hash or to publish something encrypted and share the key/password when you want to reveal the secret.
- Publicly accessible.
- Timestamped.
- Immutable (or at least with edits marked as such).
- Widely trusted (or too big to be bribed in small cases, e.g., Google).
- And keep those features for many years.
Twitter was surprisingly good at that in the past, but no more. Blockchains, as mentioned in other comments, give excellent immutability; but the field is such a minefield that I'd struggle to find a trustworthy blockchain explorer.
I didn't know. Thanks for the heads up.
Judges can be aided by expert reports.
And not all judges are non-technical.
The fact that you can defend your documents with timestamps is often enough: the other side won't challenge them knowing that they are likely to lose the challenge.
There isn't any way to do this without one or more trusted third parties. Traditionally that would involve someone like a public notary or a lawyer.
I was amused to find that there is a service that cryptographically timestamps things over email via PGP that has been running since 1995: