title: Subtracting seconds with moment.js
body: <p>I have a clock i want to start at 5:00.  and I just want to make a button for instance that would subtract 5 seconds so that it displays 4:55.  I'm having so much difficulty with this and JS DateTime.  I keep getting the error "format is not a function".  What am I missing?</p>
  
  <pre><code>var dateTime = new moment(300000);
  
  var clock = dateTime.format('m:ss');  //Displays as 5:00
  
  //Button Click
  $scope.rewindClick = function () {
    clock = dateTime.diff(5000).format('m:ss');
  }
  </code></pre>