I would just do something really simple and just hard enough such that non-programmers wouldn't be able to figure it out.
I would something like write to a file the number of milliseconds when the program was first installed in a 64-bit long in binary to a file. And have your main class check and enforce the time limit. Yes people can change their clocks to work around this, but really you don't want to sell to those people anyways. Also ensure the current time is strictly within 30 days of the install. Most users will just set their clocks back one year and it works with most programs because they just did a simple less than the difference check. You should also check that the difference in number of days from current to install is also greater than 0.
If you feel you need more protection than that, then you have a business model problem more than a software one. Its basically impossible to make it unhackable, especially since you can just extract and disassemble the class files.
