正如Martin在评论中所说，计时器的分辨率为50-100毫秒（0.05至0.1秒）。尝试以短于该间隔的时间运行计时器将无法获得可靠的结果。另外，计时器不是实时的。它们取决于它们所附加的运行循环，并且如果运行循环变得繁忙，则计时器的启动会延迟。
不要在每次触发计时器时尝试增加计数器，而是在启动计时器时记录开始时间，然后做一些数学运算以弄清楚发生了多少时间：
var startTime: NSTimeInterval//Sart the timerstartTime = NSDate.timeIntervalSinceReferenceDate()NSTimer.scheduledTimerWithTimeInterval(0.02, target: self, selector: Selector("advanceTimer:"), userInfo: nil, repeats: true)//And your timer method...func advanceTimer(timer: NSTimer){ //Total time since timer started, in seconds self.time = NSDate.timeIntervalSinceReferenceDate() - startTime //The rest of your pre goes here }编辑：
此代码的Swift 3版本如下所示：
（写为测试项目中的视图控制器）
class ViewController: UIViewController { weak var timer: Timer? var startTime: Double = 0 var time: Double = 0 @IBOutlet weak var timevalueLabel: UILabel! override func viewDidAppear(_ animated: Bool) { startTime = Date().timeIntervalSinceReferenceDate timer = Timer.scheduledTimer(timeInterval: 0.05,target: self,selector: #selector(advanceTimer(timer:)),userInfo: nil,repeats: true) } //When the view controller is about to disappear, invalidate the timer override func viewWillDisappear(_ animated: Bool) { timer?.invalidate() } func advanceTimer(timer: Timer) { //Total time since timer started, in seconds time = Date().timeIntervalSinceReferenceDate - startTime //The rest of your pre goes here //Convert the time to a string with 2 decimal places let timeString = String(format: "%.2f", time) //Display the time string to a label in our view controller timevalueLabel.text = timeString }}
