title: How to calculate and format a period of time in NSDate
body: <p>I´m assembling an application with regular web updates. Every update leaves an unix time stamp. Now i try to create an editor, which shows the elapsed time since last update. I´m convering the unix time stamp with this code:</p>
  
  <pre><code>import Cocoa
  
  struct UnixConverter {
  
      static func converUnix(_ timestamp: Int) -&gt; NSDate? {
  
          let date = NSDate(timeIntervalSince1970: TimeInterval(timestamp))
  
          return date
      }
  } 
  </code></pre>
  
  <p>Now I´m getting a time in this format: "2017-06-16 17:47:45 +0000"
  The following function gives me a reals time push update inside a <code>NSTextFieldCell</code>:</p>
  
  <pre><code>import Cocoa
  
   class ViewController: NSViewController {
  
      var timer = Timer()
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          // This function pushes the time NSTextFieldCell
          timer = Timer.scheduledTimer(timeInterval: 1.0,
                                          target: self, s
                                          elector: #selector(tick),
                                          userInfo: nil,
                                          repeats: true)
      }
  
  
      // This is the NSTextFieldCell formatter/ receiving container
      @objc func tick() {
          timerTextField.stringValue = DateFormatter.localizedString(from: NSDate() as Date, dateStyle: .medium, timeStyle: .medium)
      }
   }
  </code></pre>
  
  <p>Now "all" I would have to do, is to make a subtraction to get the difference. At this point I must confess I don´t have any idea how to do this. After that the time difference has to be pushed inside the  <code>NSTextFieldCell</code>.
  I would be very glad if someone could give me a hint how to do it. </p>