What is the best way to make a 15-day demo?

Yes I've used the "search" button. But the codes that I found are not fool-proof.


My current method is to store the encrypted installation date in the registry. And whenever user opens the program, use the DateDiff function to see if it is over 15 days from the date of installation.


Apparently, that method has many holes.

1. I can change my comp date to something like "1/Jan/2337" before installing the program. After installaion, I change it back to year 2007. I will be able to use the demo program forever.

2. I can roll back to the installation date and keep using the demo forever..

How to prevent these?