title: Why does moment.js subtract method not works?
body: <p>I am trying to subtract 7 days to a given date using moment.js</p>
  
  <pre><code>var date ="2015-10-19";
  var now =  moment(date);
  var oneWeekAgo = moment(date).subtract(7,'days');
  </code></pre>
  
  <p>When I check values of <code>now</code> and <code>oneWeekAgo</code> have the same content both.</p>
  
  <pre><code>Moment {_isAMomentObject: true, _i: "2015-10-19", _f: "YYYY-MM-DD ", _isUTC: false, _pf: Object…}
  </code></pre>
  
  <p>However, If I subtract 7 days to current time it works.</p>
  
  <p><code>var oneWeekAgo = moment().subtract(7,'days');</code></p>
  
  <p>I've noticed that instanciate a moment without arguments change his structure, storing date on <code>_d</code> attribute instead <code>_i</code>.</p>
  
  <pre><code>Moment {_isAMomentObject: true, _isUTC: false, _pf: Object, _locale: Locale, _d: Tue Oct 13 2015 13:34:50 GMT+0200 (Hora de verano romance)}
  </code></pre>
  
  <p>Why it happens? and how can I solve it?
  Thanx. </p>