title: Licensing system with expiration date
body: <p>I want to implement a simple licensing system based on <a href="http://www.codeproject.com/articles/99499/implementation-of-the-licensing-system-for-a-softw" rel="nofollow">this article</a>.</p>
  
  <p>Everything works fine. But now I want to add a expiration date and I do not know how.</p>
  
  <p>Can someone explain me how to add the expiration date? It is not important to know exactly how to implement, but I need to understand the algorithm behind it :)</p>