title: Stop watch and timer
body: <p>I want to use NSDate and NSTimer to retrieve the elapsed time between I press a start button and a stop button and present the time in seconds. Then I want to use the time for a calculation. I know how to get the time but it is always in format like this 2013-01-15 14:01:55.369.
  How do I subtract one time from the other to get the seconds.
  I am not sure how to use NDDate and NSTimer.
  This should be really easy but I am sort of stuck here.</p>