date can be easily changed as you know thats why i need to check it via servers =) The app is an online app so they need to be connected to internet so no problem for them.
If it is connected then you can get the time from a internet site so changing it on the phone wont do much.
But i would still prefer using an offline solution.
I would store the last purchase date as a variable and also on every start store the datetime.now as laststart variable
Now on every start and resume check if datetime.now > 1week
If true disable full version if false make another check
If datetime.now > laststart
If false user changed his clock in his phone now you can again disable full version
You may have 1 problem witb this method. If user will travel to another country and change his clock or if wintertime changed.
So i guess the check online time would be simpler and more accurate.