You could have another registry key that you increment after every day’s use. That way, even if they change the computer’s date, this key would indicate to your program that it’s been running for > 30 days.
Additionally, this value could be encrypted so that if the user tries to manually change it, the program can refuse to run because it was unable to decrypt the value and get a valid number out of it.
To get around reinstalls, you could add some information to any file saved with the trial version of your app which is unique to that specific version of the app (perhaps a timestamp from when it was installed). When a trial version of your app tries to open a file, it will check this signature and ensure that it was created with that same instance, otherwise refuse to open the file. This essentially neuters the ability to simply reinstall the app and continue using it.
At the end of the day though, the user has complete control over their machine and can probably find a way around whatever it is you want to do (short of accessing a web service where these details are kept before you let the user use the app). You probably shouldn’t expend so much energy trying to stop the guys who are willing to go through this extra trouble, but instead spend that extra time/money/energy improving the app for those who are willing to pay.