title: Prevent countdown going into negatives
body: <p>I've created a countdown timer and it works great.. but when it reaches zero it continues counting down.. so it shows -1, -2, -3 etc. How do I prevent it from doing this?</p>
  
  <p>This is my code from the implementation file..</p>
  
  <pre><code>@implementation ViewController
  
  -(IBAction)start {
  
      myTicker = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(showActivity) userInfo:nil repeats:YES];
  }
  
  -(IBAction)stop {
      [myTicker invalidate];
  }
  
  -(IBAction)reset {
      time.text = @"0";
  }
  
  -(void)showActivity {
      int currentTime = [time.text intValue];
      int newTime = currentTime -1;
      time.text = [NSString stringWithFormat:@"%d", newTime];
  }
  </code></pre>
  
  <p>I'm guessing that I'm missing some code somewhere?</p>