TrialWare for my application

Question

I have an application that I've written using C#. I want to incorporate trialware functionality so that a user can run ym program for X many days. Any suggestions on this? I don't consider simply reading/writing to the registry an acceptable solution. Nor do I like the notion of a license file. Perhaps some combination of these would prove an adequate solution. I want to prevent the following: 1. user setting back the clock to get more free trial. 2. Uninstall - reinstall to get a new free trial 3. hacking a file or registry to turn off the trial. <- I don't think this is likely, given the intended customer base. Thanks for your help.