请检查这个答案我希望它能解决你的问题

//
//  SampleStopWatch.swift
//

import UIKit

class SampleStopWatch:UIViewController {
    
    var timer:Timer?
    var startTime = Date()
   
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        timer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: (#selector(updateTimer)), userInfo: nil, repeats: true)
    }
    
    @objc func updateTimer() {
        let timeInterval =  Date().timeIntervalSince(startTime)
        let titleLabel = timeInterval.stringFromTimeInterval() // show this text on label
        //labeltoShow.text = titleLabel
        print("title label " + titleLabel)
    }
}
extension TimeInterval{
    
    func stringFromTimeInterval() -> String {
        
        let time = NSInteger(self)
        
        let ms = Int((self.truncatingRemainder(dividingBy: 1)) * 1000)
        let seconds = time % 60
        let minutes = (time / 60) % 60
        let hours = (time / 3600)
        
        return String(format: "%0.2d:%0.2d:%0.2d.%0.3d",hours,minutes,seconds,ms)
        
    }
}

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