你可以不使用后台计时器来做这样的事情。这只是你如何达到你的要求的想法，根据你的要求添加一个或多个小时的条件。

var totalTime = Double()

override func viewDidLoad() {
  super.viewDidLoad()

// MARK: - To Reset timer's sec if app is in background and foreground
    NotificationCenter.default.addObserver(self, selector: #selector(self.background(_:)), name: UIApplication.didEnterBackgroundNotification, object: nil)

    NotificationCenter.default.addObserver(self, selector: #selector(self.foreground(_:)), name: UIApplication.willEnterForegroundNotification, object: nil)
}

 @objc func background(_ notification: Notification) {
   if self.totalTime > 0{
     user_default.setValue(self.totalTime, forKey: "TotalSecInBackground")
     user_default.setValue(Date().timeIntervalSince1970, forKey: "OldTimeStamp")
     LogInfo("total seconds left in background: \(self.totalTime)")
  }
}

  @objc func foreground(_ notification: Notification) {
   let timerValue: TimeInterval = user_default.value(forKey: "TotalSecInBackground") as? TimeInterval ?? 0
   let otpTimeStamp = user_default.value(forKey: "OldTimeStamp") as? TimeInterval ?? 0
   let timeDiff = Date().timeIntervalSince1970 - otpTimeStamp
    if timerValue > timeDiff{
    LogInfo("total second & timeDiff:, \(Int(timerValue)),\(Int(timeDiff))")
    let timeLeft = timerValue - timeDiff
    self.totalTime = Int(timeLeft)
    LogInfo("timeLeft: \(Int(timeLeft))") // <- This is what you need
}}