You can just publish encrypted text as a GitHub gist (or use any other large public site where you can't modify the timestamps), and then release the decryption keys later.

Or only publish a hash that used some collision-resistant function (future proof), and then publish the text that matches the hash later. For this, you can just tweet it, or write it into a HN comment.

$2a$12$0qd0ztFAO5XAjQ.HRDMxjeS3n6tsemeHfWfWO0MIBR0mwFfmyp2RO