>> "For verified trials, are there any protection against system date change?"
Yes. We don't go into detail about all of the client-side fraud detection we do.
>> "I'm able to change the system date on my Windows 7 machine and get around the trial expiration ie I can set it to an earlier date and continue with trial even though it expired as per real date."
The upcoming 4.1 release further hardens against a slew of rollback type fraud edge-cases.