How to make Trial Version

hi
i want to make trail version of my ASP.NET web application project. I got information from google regarding this subject where most of solutions have been given by using current date of server. But i want to check that if user change date time e.g if my application has only 5 days left and if user change the date (change to old date)of server, application will run again until date will reach which i have to expire and definitely that date will never come if user change the date to backward.
 
any idea 
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