One more thing.

Consider publishing all you hashes for each day in a digest to e.g. USENET or other permanent public storage, then hashing all of those daily hashes together and publishing the meta-hash in New York Times. That way even if blockchain fails as a dating method due to forgery, one could use USENET + paper copies of NYT to date any of the documents. As a matter of fact, there is no limit to how many different ways of dating a hash you could add to your service.

Just make sure you're also including a previous day's unpredictable event (stock market activity?), so that both early and late boundaries can be established with confidence.