Windows form applicaion with expiration date

Question

Hello

I've developed C# application in which needs to have an expire date.
It's a simple windows form application which users can run exe file for a limited time only. Maybe later I might need to go with installer for my application. Anyhow,  The application should stop working after 90 days for instance or maybe on any specified date (20th of June 2014 ) for example.

What is the right way to get time stamp for my application? I think I need to write C# function which compare Dates and times. I've read that xml file can be used or maybe to write to registries. I have no idea from where to start. Can you provide me with some practical solution? or maybe the write code to achieve this?