I last flew on Frontier on Jan 31. I looked at my account and saw that the expiration date was shown as Sept 30 (should have been July 31). I emailed them (through the "contact us" form) to get clarification. Their reply:
Please attach a copy of your account showing the expiration date of September 30, 2016 on your email response.
I sent a screenshot of my account page showing the date (I can't believe they couldn't look it up themselves somehow. There reply was:
I have reviewed the supporting documentation. As a courtesy, we have honored the expiration date reflecting on your profile.
I think I'll find away to accrue miles before July 31 anyway. Looking forward to using up my miles.