title: Android: How to detect if user changes system time without running a constant service to check time
body: <p>In my app I want to allow user to be able to use it for 7 days offline usage for free, but want him to connect online once 7 days are done and buy a subscription after that. So basically, I want to lock him out after 7 days from install. The problem is user can very well change system time and trick the app. Also, I cannot use server as the authority as the app is supposed to also work completely offline, except for the time of purchasing subscription. </p>
  
  <p>I can run a background service to keep track of any system time change but I want to avoid doing that.
  Is there a straight forward solution which can work even if the device is totally offline for 7 days.</p>