I suspect that means that the file hash is published to a trusted third party who then combines the hash with a timestamp:<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Trusted_timestamping" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Trusted_timestamping</a>