title: What's the minimum valid time interval of an NSTimer?
body: <p>I want to use <code>NSTimer</code> to increase the number which show on a label.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  
  self.numberLabel = [[UILabel alloc]initWithFrame:CGRectMake(90, 90, 90, 30)];
  [self.view addSubview:self.numberLabel];
  
  
  self.timer = [NSTimer scheduledTimerWithTimeInterval:0.01 target:self selector:@selector(refreshText) userInfo:nil repeats:YES];
  [[NSRunLoop currentRunLoop]addTimer:self.timer forMode:NSRunLoopCommonModes];
  }
  
  - (void)refreshText{
  
  NSDate *beginDate = [NSDate date];
  
  static NSInteger a = 0;
  a ++;
  
  self.numberLabel.text = [NSString stringWithFormat:@"%ld",a];
  
  if (a == 1000) {
  
      NSDate *endDate = [NSDate date];
  
      NSTimeInterval durationTime = [endDate timeIntervalSinceDate:beginDate];
      NSTimeInterval intervalTime = self.timer.timeInterval;
  
      NSLog(@"durationTime = %f",durationTime);
      NSLog(@"intervalTime = %f",intervalTime);
  
      [self.timer invalidate];
      self.timer = nil;
  } 
  }
  </code></pre>
  
  <p>and the console showed:
  <a href="https://i.stack.imgur.com/GRz3e.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GRz3e.png" alt="enter image description here"></a></p>
  
  <p>then I changed the timer's timeInterval from 0.01 to 0.001,the console showed:
  <a href="https://i.stack.imgur.com/y7Tyn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/y7Tyn.png" alt="enter image description here"></a></p>
  
  <p><strong>What confused me is that why the durationTime is not 0.0000056 when the timeInterval is 0.001.What's more,is there a min value for NSTimer's timeInterval we can set?</strong></p>