How do you reliably prove when data existed?

I keep running into the same problem across many domains (documents, AI outputs, datasets, code, logs).

Storage tells me what I have now. Versioning tells me what changed.

But when a dispute happens, the real question is often simpler: When did this data actually exist?

Screenshots, PDFs, dashboards, emails… they mostly rely on trust in a system or platform.

In many cases, storing or sharing the original data isn’t even an option (privacy, compliance, scale).

I’m curious how others handle this today. What approaches have you seen work in practice?