I'm inclined to agree with @mrvisser as it makes slightly more sense from a model standpoint.
Passing in the full messageId (messageBoxId:created) seems a bit bloated to me as you already need to pass the discussionId (which will be nearly the same thing as the messageBoxId) as the top resource ID.

My vote would go for passing the created timestamps around.

(FWIW: Revisions is the opposite of this, we pass the revisionId (rev:cam:abc123) around when operating on revisions.)