.net licensing and expiration dates

[roy]

Hi, i'm working in a license scheme for my desktop aplications, I'm using signed xml for licenses, my problem is how to implement an expiration date, because I can't trust the system date (the user can easly changed it). From where and how I can get a current date that I can compare against the expiration one ?

A simple solution is to use web service, but I can't take for granted an internet connection in my clients. There's another way ?

Thanks