title: $filter('date') returning "invalid date"
body: <p>I have a date string formatted like this:</p>
  
  <pre><code>20150917
  </code></pre>
  
  <p>I want to convert it to the format</p>
  
  <pre><code>17-sep-2015 
  </code></pre>
  
  <p>So I am using this code to first convert the former to a date object and then convert it back to a string with the new format:</p>
  
  <pre><code>var myDate = $filter('date')(new Date(dateStr, 'yyyyMMdd'));
  console.log(myDate); //to make sure
  </code></pre>
  
  <p>that console.log is giving me </p>
  
  <pre><code>Invalid Date
  </code></pre>
  
  <p>why?</p>