title: toLocaleString doesn't seem to work as expected
body: <p>I am trying to print out a date/time in a format that the user is comfortable with. I have:</p>
  
  <pre><code>options = { day: "numeric", year: "numeric", month: "short", time: "short", hour12: false, hour: "2-digit", minute: "2-digit" };
  </code></pre>
  
  <p>When the <code>locale</code> is specified explictly, <code>options</code> appear to be followed:</p>
  
  <pre><code>date.toLocaleString("en-US", options);
  </code></pre>
  
  <p>will produce:</p>
  
  <blockquote>
    <p>Jun 26, 2017, 13:05</p>
  </blockquote>
  
  <p>If I leave out the locale parameter:</p>
  
  <pre><code>date.toLocaleString(options);
  </code></pre>
  
  <p>will produce:</p>
  
  <blockquote>
    <p>6/26/2017, 1:05:00 PM</p>
  </blockquote>
  
  <p>What is the locale that will be used if I leave it out? As far as I know, the locale of my PC and browser used for the test is en-US. </p>
  
  <p>How do I display date/time using the browser's locale without hardcoding?</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var options = { day: "numeric", year: "numeric", month: "short", time: "short", hour12: false, hour: "2-digit", minute: "2-digit" };
  
  var D = new Date();
  d = D.toLocaleString("en-US", options);
  e = D.toLocaleString(options);
  f = D.toLocaleString("en-US");
  
  $("#d").text(d);
  $("#e").text(e);
  $("#f").text(f);</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
  
  &lt;div id='d'&gt;&lt;/div&gt;
  &lt;div id='e'&gt;&lt;/div&gt;
  &lt;div id='f'&gt;&lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>