A "trial period" (aka 'time bomb') could be implemented, but it would still require Excel macros to create and verify dates, and therefore would be easy to circumvent for anyone with Excel and Google since they could get into the code to see what it does before the trial period is up.

As mentioned, there aren't really any secure parts to Excel. You could compile all of your code into a DLL file, and simply pass data back and forth between Excel and your functions and not rely on any in-sheet formulas or code, but even DLL's can be decompiled (although it would keep the vast majority of users out of your functions).