title: how to clear NSTimer memory?
body: <p>I'm using <code>NSTimer</code> to display seconds on a label while recording.
  I'm starting the timer when the record button is tapped and stopping it when the stop button is tapped.
  When I tap the play button the timer is not restarting.</p>
  
  <pre><code>-(void)startTimer
  {
   playTimer = [NSTimer scheduledTimerWithTimeInterval:1
                                               target:self
                                             selector:@selector(timerController)
                                             userInfo:nil
                                              repeats:YES];
  }
  - (NSString*)getTimeStr : (int) secondsElapsed
  {
  seconds = secondsElapsed % 60;
  int minutes = secondsElapsed / 60;
  int hours = secondsElapsed/3600;
  return [NSString stringWithFormat:@"%02d:%02d:%02d", hours, minutes, seconds];
  }
  
  - (void)timerController{
  seconds++;
  [[self timeLabel] setText:[self getTimeStr:(seconds)]];
  }
  
  - (IBAction)recordTapped:(id)sender {
  [self startTimer];
  }
  
  - (IBAction)stopTapped:(id)sender {
   [playTimer invalidate];
    playTimer=nil;
   _timeLabel.text=@"00:00:00";
  }
  
  - (IBAction)playTapped:(id)sender {
  [self startTimer];
  }
  </code></pre>