title: Time run down slower in Spritekit update function
body: <p>I'm making a game in Spritekit, and there's a timer going down that keeps changing in the update function. Sometimes in my game, I change the speed of the physics world, and when that happens I would like for the timer that's going down to also change speed.
  To be more specific, I'm changing the speed to 0.5.
  So I'd like, in that period for the time to run down at half the speed it's going by.
  Here's my code for running down the time. lastSwitch is called in the didMove function.</p>
  
  <pre><code> var lastSwitch = CFAbsoluteTimeGetCurrent()
   override func update(_ currentTime: TimeInterval) {
      let currentTime = CFAbsoluteTimeGetCurrent()
  
      let timePassed = currentTime - lastSwitch
  
      let timeLeft = totalTime - timePassed
      //... bla bla
      time.text = "\(timeLeft)"
   }
  </code></pre>