First off, don't use the registry.
The registry in Vista and above need admin access to modify, so every time you call SetNewDate you app will require UAE access, and this can only get more rigid in future versions.
Instead, create a licence file and store it somewhere more accessible:
Where should I store my data?[
^] explains better locations.
I'd look at what you stored myself: start with putting a breakpoint in the StoreDate method and see how often it is called, and with what values.