title: How to validate timestamp in Firebase?
body: <p>In my chess app I use <code>Firebase.ServerValue.TIMESTAMP</code> to record the time of a move, e.g.:</p>
  
  <pre><code>{
    from: 'e2',
    to: 'e4',
    playedAt: Firebase.ServerValue.TIMESTAMP
  }
  </code></pre>
  
  <p>How could I validate that <code>playedAt</code> is <code>Firebase.ServerValue.TIMESTAMP</code> and not an arbitrary timestamp like 1408663907743?</p>
  
  <p>I expected Firebase Security Rules to have a method like <code>isTimestamp()</code> that would work similarly to <code>isNumber()</code> and <code>isString()</code>, but I couldn't find one.</p>