I strongly suggest that you keep the timestamps outside of (separate from) whichever digest function you use. Digests are inherently unorderable with respect to time.
I've been a little busy this week so didn't bother looking back at this thread, but my (unstated) reasoning behind putting the timestamps inside the hash was for easy sortability on a single field, at the expense of easy time ordering. It also guarantees that short messages that may be have dupes in the past or future get unique digests. It might be reasonable to include the timestamp both inside the digest and outside as a separate column, depending on the application.