title: Timer that updates label running in background
body: <p>I have coded a timer using an NSTimer that updates a label. The problem is that in the same viewcontroller I have a uitableview and when I scroll it down, the timer doesn't update its value so the user can "cheat" to stop the timer.</p>
  
  <p>I think this could be easy to fix with a serial queue with CGD but I don't figure out how to do it.</p>
  
  <p>Thanks in advance,</p>
  
  <p>Juan</p>