Comment by XorNot

You literally just need several oracles which sign hashes at the time they receive them and record that fact.

As a community service you need them to have enough scale that no individual hash or source can be tampered with without being likely to become known as unreliable to everyone else as well ala certificate transparency records.

(You could probably just bootstrap let's encrypt for this - issuing a certificate you use to sign a bunch of data would stamp several minimums on the order anything could have happened).