I setup a couple of test accounts in my blog. I put them in a group and gave each an expiration date of 09-11-2012 (today). I can still access the site and pages using the two test accounts that were supposed to have expired. I'm confused about how you're defining "expiration" if the accounts are still active.
When the expiration date hits, are the accounts with that expiry date locked down (i.e. password wiped/changed) or deleted?