title: how to keep running NSTimer when app goes in background
body: <p>I am creating puzzle game application and in that I am displaying time  (i.e 01:20) using NSTimer. NSTimer is paused when application gone in background but i want to continue it even application is in background state.</p>
  
  <p>for example <strong>timer count is 15 sec when app gone in background</strong> and I <strong>put it for 5 sec</strong> and become in foreground now I <strong>need to timer count changes to 20 sec</strong> </p>
  
  <p>I have searched a lot but didn't get good answer.
  So Please suggest me how can I achieve this.</p>