expire product after 15 days trial

i want that my product should expire after 15 days.
what i am doing is i am creating a registry file where i am writing the installation date and after 15 day i am expire the product.
but my issue comes when user changes the system date and time then my approach fails.
kindly suggest the way which i can do.
dont what to do with some licence encrypte dfile.
suggest someting with registery issue