title: Clearing NSUserDefaults after specified time period iPhone
body: <p>I have created NSUserDefaults object, it will be updated with the new value whenever an even will occur. 
  What I want is (as per my application requirement) that object should be cleared every 7 days. 
  Like if 1st time that NSUserDefaults are updated today so exact after 7 days, a method should work and clear the NSUserDefaults. 
  So that, new values would be assigned from then on for the next 7 days.</p>
  
  <p>Is it possible in objective-c?</p>