One potential subproblem is how to set a time-release message that was verifiably sent over 24 hours ago and not read since being sent. Embedding a reliable secret in such a message would then authenticate it as yours (if you had such a secret).

My first thought is to post the message publicly, encrypted, with the intention that it be brute-forced. The viability of this depends on the adversary's compute resources compared to yours, and how long you're willing to wait before being able to read the message.

Otherwise you could try to pursue physical means. If you had the resources, maybe you could send something into space and back? Otherwise, what you need is a seal such that you can verify how long it's been sealed. This probably exists, but it comes down to materials science, which I don't know well. Maybe if you suspend the message in water and put it in a freezer, you'll be able to verify that it's been freezing for a certain duration at a certain temperature?