Setting expriy date. for windows application

Created one windows applcation.

now i would like to give licence for it by keeping expiry date.

am planning to keep expiry date in registries.

the porblem is..

how can i get the actaul date. ie; when user changes the system date also- this should not change.(i meran expiry date.) Am looking for logics to acheive this..!!

date will be encrypted开发者_如何学Python with the encryption algorithm- defined by our team.

And what will stop a user to go into the registry and change these values? There are many savvy users out there.

Perhaps you should use a license manager, like here or here (just two links off google).