Advice for dated archives

[Harry Putnam]

Summary: Looking for a smooth way to allow rsyncing of mail and news
         to generate cumulative archives, but cut off a specified
         date, and begin a new archive that contains nothing of the
         previouis one.

That might have sounded kind of confusing, but here's my setup: I
routinely rsync my ~/Mail ~/News (news is downloaded usenet) to
cumulative archives so that as expiration/expunge etc occur in ~/Mail
~/News I have the current working(trimmed down) set but also a large
archive of everthing since (whenever).  /some/archive/Mail
/some/archive/News.

As you may imagine the archives eventually grow unusably large.  I'd
like to figure out a cool accurate way to continue this setup but
generate a new set of archives (keeping the old) with dated names.
/some/Mail/archive_start_DATE ...

Trouble is how to sync the current trimmed down set with the new
archive, so that none of the messages in current ~/Mail(News) that
are already in the previous archive, get rsynced to the new one.  So
I have a clean break with no duplication.

Something similar to the `newer' option to find command would be nice.
So that one day I could just tell rsync to move stuff to the new
archive but not if its older than DATE.

Any ideas how to accomplish this with the least huffing and puffing?

I did try some experiments with --compare-dest=DIR but didn't really
see how to accomplish the above goal with that.