If you want to prove that "happened at or after this timestamp" you can use a randomness beacon. NIST[0] and others publish a random number every N minutes. Embed that (or a combination) of those seeds to prove that you observed this value. This does not work for the harder problem of proving an event happened before a timestamp.

[0] https://csrc.nist.gov/projects/interoperable-randomness-beac...