how to check expiry date even system date changed

Dear Friends,

Greetings! iam creating an app with Adobe Air for IOS and SQLite, I have list of buttons here, After a certain period, I want to disable a particular button. I can compare system date and make it disable that buttons. Its working fine. But if the user changed system date before that expiry date, how can I track and control?, User knows his expiry date all the time, so he can change system date one day before his expiry date and make that button to enable. Please help me to track and control this expiry date of that button click even user changed system date.

Thanks in advance,

Regards,

Syed Abdul Rahim