You could use a trusted / centralised source to produce a TOTP or some equivalent code, and generate some code alongside a timestamp, which would be verifiable.

You could use a trusted / centralised source to produce a TOTP or some equivalent code, and generate some code alongside a timestamp, which would be verifiable. (You can't get the code ahead of time from the service, but can later verify a timestamp code combination.) Honestly, just emit the message in the Bitcoin blockchain and it will be there for perpetuity with a timestamp that is fully verifiable.