title: ios: find real time since previous app launch
body: <p>SO basically all is in the title. I've searched quite a lot, but didn't find any right solution which doesn't require internet connection. 
  If the user changes time in settings - i can't find real time since last launch. </p>
  
  <p>I need that for my game, in it for every hour, even when you don't play the game, you get some coins.</p>
  
  <p>If the user changes time in settings - that affect the time in NSDate() and user can cheat with coins.</p>