Simple implementation of a "clock" or "stop watch", capable of been paused

AlarmClock.swift

import Foundation

class AlarmClock: SimpleClock {
	
	var timeout: TimeInterval = 0
	
	var hasExpired: Bool {
		return duration >= timeout
	}
	
	var timeRemaining: TimeInterval {
		return max(timeout - duration, 0)
	}
	
}

SimpleClock.swift

import Foundation

public class SimpleClock {
	
	internal var startedAt: Date? = nil
	internal var totalRunningTime: TimeInterval = 0 // Used for pause/resume
	
	var isRunning: Bool = false {
		didSet {
			if isRunning {
				startedAt = Date()
			} else {
				totalRunningTime += currentCycleDuration
				self.startedAt = nil
			}
		}
	}
	
	// This is the amount of time that this cycle has been running,
	// that is, the amount of time since the clock was started to now.
	// It does not include other cycles
	internal var currentCycleDuration: TimeInterval {
		guard let startedAt = startedAt else {
			return 0
		}
		return Date().timeIntervalSince(startedAt)
	}
	
	func reset() {
		isRunning = false
		totalRunningTime = 0
	}
	
	// This is the "total" amount of time the clock has been allowed
	// to run for, excluding periods when the clock was paused
	var duration: TimeInterval {
		return totalRunningTime + currentCycleDuration
	}
	
}