As noted, time-based licensing is pretty easy to get around. You can jump through some hoops to keep people from resetting their system clock. You could contact a certified time server and set your internal clock that way, but it wouldn't take a rocket scientist to crack that if they can access your code. An easy way is to just find the time check in the asm listing and branch around it.
We've worked on this for years (disclosure: I work for a copy protection company (www.wibu.us)) and use a combination of an internal clock on a smart card chip and certified time servers, plus some code to make sure that you can never set the time back (code is always encrypted so it's not patchable). We also have a software-only solution which uses an internal clock but not on a smart card chip. There are drawbacks to all security measures; finding the right tradeoffs for your market, price point, etc are the trick.