Yes, it's only for the dates specified. In order to check if you have any offers attached to your name, you can either call and hope that the CM will actually check for you (keep calling until you get a good one) or you can check the site map on the Disney website. There is something there called "my offers" that you can click and see if there's anything there for you.
I didn't get a postcard, but I just checked my online profile and they still have my old address from 2 years ago, so I'm betting I won't see one even if they mailed one out to me. I can't seem to find the "my offers" on the Disney site after I sign in. Can anyone point me in the right direction?
Thanks.