title: md-datepicker must be a Date instance Error
body: <p>I have a problem here. I want to use Angular Material datepicker in my modal form. I simply added it to my form like this</p>
  
  <pre><code>&lt;md-datepicker ng-model="new.warranty" md-placeholder="Enter date"&gt;&lt;/md-datepicker&gt;
  </code></pre>
  
  <p>And if I submitting form I got this:</p>
  
  <pre><code>Sun Apr 03 2016 00:00:00 GMT+0300 (EEST)
  </code></pre>
  
  <p>But I want to convert data in yyyy-MM-dd format before send, so I tried to convert date format in this way</p>
  
  <pre><code>data.warranty = $filter('date')(Date.parse(data.warranty), 'yyyy-MM-dd');
  </code></pre>
  
  <p>So my form rly send a required yyyy-MM-dd format, but I get an error every time, when I sumbitting form.</p>
  
  <pre><code>`The ng-model for md-datepicker must be a Date instance. Currently the model is a: string`
  </code></pre>
  
  <p>so, how can I prevent this error, without using moment.js, if it possible, please.</p>