title: Date.now().toISOString() throwing error "not a function"
body: <p>I am running Node v6.4.0 on Windows 10.  In one of my Javascript files I am trying to get an ISO date string from the Date object:</p>
  
  <pre><code>let timestamp = Date.now().toISOString();
  </code></pre>
  
  <p>This throws:
  Date.now(...).toISOString is not a function</p>
  
  <p>Looking through stackoverflow this should work...possible bug in Node?</p>