How to protect software from system date-time changes?
I would like to add licensing system to application. For example: user buys license for 1 month and after that program expires (Kinda Anti-Virus style?).
Problem is that application is supposed to run in systems which may or may not be connected to internet, so how to protect from date-time changes?
Storing app startup and close times in encrypted file won't work as date can be changed (with program uptime of 8 hours per day, would be possible to extend license to almost 300% in ideal case - change time to app close time + 1 second before launching program).
Another question - is there any way to protect from software like http://en.wikipedia.org/wiki/Deep_Freeze_(software)? (maybe scan drivers?)
EDIT:
I'm currently using smart card to store licensing information and will use code virtualizer on critical functions (I know about making breakpoints on API calls and inspecting passed data - don't need to hide that data, just to ensure things go as planned)
Answer
Yes, it would be possible to extend the license by up to 300% but at great effort to the user. Frankly if someone is going to spend every day of a month resetting their time to one second after they quit your program before starting it again to use it longer, there is nothing you are going to do about it, and the time you take to stop them will cost you more than they ever will.