What about inserting such a line:
if now > datenum('15-Dec-2012'), return; end
Then the user can still reset the clock, but there is no fair method to block this.
Another method is to check the existence of a certain webpage and delete it when the license expires. Then the internet traffic can be observed and the existence of the page can by simulated with less than half an hour of work. An encrypted communication with a webserver would be more secure. But remember, that such dongles are hated by users, because they block the trial software in case of tiny net connection problems already. This is not a good advertisment.
A big red banner on top of the figures might me better: If the evaulation tome has expired, show a large "THIS IS A TRIAL VERSION ONLY" string in the figures. The user can still run the software such that he does not feel to be foreced to cheat. But the motivation to pay is higher.