You can have sort of licensing feature in your application.
There are different ways to achieve this
1) encrypted license files that contains the expiry dates etc.
2) encrypted license keys held in the registry
3) challenge/response mechanism which require an internet connection
4) probably others.......
Try: License Key Generation
] or: How can I create a Product Key for my C# App
] or: c# Licensing
Cryptography 101 for the .NET Framework