title: Group by date from timestamp in mongoose
body: <p>I'm adding exact timestamp to every document. I need to display number of documents received according to date.</p>
  
  <p>I tied</p>
  
  <pre><code>{$group: {_id: '$timestamp' }},function(err,docs){}
  </code></pre>
  
  <p>But as the timestamp also includes time, each document is getting in different group.</p>
  
  <p>How do I group those docs by Date/Month?</p>