You are presumably an ex-MobileMe subscriber who has had free extra storage since last year.
If you have actually purchased an upgrade, that will have cancelled the free storage from the time you made the purchase, and it will expire a year later.
If you did not purchase an upgrade, then your free extra storage will show as if you had purchased it, and will expire on Sept 30th.
What date does it give here? -

To extend the complimentary Storage by paying for it please see this page:
http://support.apple.com/kb/HT5527
and expand 'How do I purchase my current plan' for instructions.