Make the installer put the installation date in the registry.
Not plaintext, encode it somehow, so it's not readable as a date.
Build a small .exe file reading that registry entry and returning the days since installation.
The .exe should not return anything when not called with some weird parameter,
see it as a kind of passwort to access the registry data.
Call that .exe from within WME submitting the right parameter. If the .exe returns a value > 30 or no value at all (.exe changed, registry value deleted etc), stop the trial.
Must I mention that time limited trial versions suck as much as copy protections?
I appreciate trial version, but please ... do not make them time dependend.
Make them usage time dependend ... as reflexive arcade does for games they publish.
Find out the total running time of your game, and after perhaps 10 hours of running time, stop the trial.
If for some weird reason the user happens to not have time to spend on the trial, the trial is wasted, for he cannot spend his time afterwards. In a usage time based trial, he can use the trial whenever HE wants, and still have a time limit.
Please think about that