title: How can convert string to date with mongo aggregation?
body: <p>In a collection, I store this kind of document</p>
  
  <pre><code>{
      "_id" : 1,
      "created_at" : "2016/01/01 12:10:10",
      ...
  }.
  {
      "_id" : 2,
      "created_at" : "2016/01/04 12:10:10",
      ...
  }
  </code></pre>
  
  <p>I would like to find documents have "creared_at" > 2016/01/01 by using  aggregation pipeline.</p>
  
  <p>Anybody have solution to convert "created_at" to date so can conpare in aggregation?</p>