Problem: Customer X is (considering) porting one of his formerly "Windows only" shareware applications to Mac OS X. One of his concerns is how to enforce the 30-day trial period and make it immune to "system rollback" software like Time Machine.
Question: What is the industry best practice for preventing the use of "system rollback" software to lengthen or otherwise modify a 30-day trial period on shareware?
DISCLAIMER: I do not consider the strategy of Customer X to be a viable one, and I don't see how people will have the incentive to rollback their whole system just to use his software for a few more days. Nevertheless, for the sake of due diligence I ask here just so I can research his question. Please consider this if you wish to down-vote merely because you (and I) may disagree or find this impractical.
How much money will he spend in finding and implementing a solution? How much money will he lose by people extending their trial period? What's the ROI?
Oh wait, that wasn't the question. He wants to do something even if it costs him more than it earns.