Relevant: https://github.com/opentimestamps/nist-inject

tl;dr: I have a long standing project that timestamps the NIST random beacon, and a bunch of other blockchains, with OTS/Bitcoin. This means that Bitcoin blocks can be proven to have been created _after_ those random beacons, giving even more evidence as to the accuracy of Bitcoin block header times.

Also, I'm time-stamping Bitcoin block headers with a bunch of Roughtime servers: https://github.com/opentimestamps/restamp Again, this provides evidence proving Bitcoin blocks were create prior to those trusted Roughtime servers.