title: How to create negative Firebase timestamp in swift
body: <p>For an iOS app I am working on, I need to fetch messages in descending order i.e the latest message comes first, followed by second newest message etc.</p>
  
  <p>From looking at other SO answers and research it seems that the best approach for my situation is to create a negative timestamp and then persist that to the database as an extra property to messages. </p>
  
  <p>I will then use <code>queryOrderedByChild('negativeTimestamp')</code> to fetch the messages in a <code>observeSingleEvent</code> and then have a childAdded observer to handle messages which are sent once initial calls are made.</p>
  
  <p>In the firebase <a href="https://firebase.google.com/docs/database/ios/offline-capabilities#server-timestamps" rel="noreferrer">documentation</a> it says I can get the server timestamp value from this snippet of code <strong>firebase.database.ServerValue.TIMESTAMP</strong></p>
  
  <p>How do I write this for Swift 3?</p>