By restricting continuous use of Windows 7 Beta to just two hours per session, Microsoft is hoping to force users to switch over to fully paid license versions.
Believe microsoft said something about that on the download page and the emails they sent, but I can't find it in any of them :/ Guess you are right then, but at least it'll save me the horror of switching back to Vista.
as long as you dont mind ur date being wrong try takin the date back to march 2000 your win7 will last ten years (dont quote me on it but when i put the date forward it said my win7 copie had expired and i had 3 days to add a valid license key so makes me think that if you put the date back it might still work!)