You should use the util.prefs.Preferences class to store your time played. This will put it in the registry for Windows, in a prefs file for macos and do something equivalent on Linux. If you want to make it even harder for hackers then you can store a second copy of the count somewhere in the filesystem. Then check them both when you start up and use the one that has counted further. Set them both the the proper count at exit.
I’ve gone on to level limit it now anyway 
You just get a license key on purchase (which now works through plimus). Enter that and the name you registered with and they get stored in a local file. I’m happy enough for registered users to copy the file about since it sticks there name on the registration screen. If anyone was to bother making a registered version available their name would be on it. I guess that’ll do?
Kev