The problem with trying to limit the dates is that the naive solution of just checking the date is easily fooled if the person sets back their system clock. I worked with a guy who kept a VMWare virtual box just for running time limited software. A better approach would be to record the time of the last time it was run, and if the time ever goes before that time, you know the clock was set back and you can abort. The problem is figuring out how to stash that file somewhere where the user can't find it an overwrite it. Again, in the the VMWare or VirtualBox environment, the user could just roll back to an earlier snapshot.
In other words, you can limit some of the people some of the time, but not all of the people all of the time.
