title: Unable to use moment.js when aggregating in Mongoose
body: <p>I am using aggregation pipeline like so:</p>
  
  <pre><code>Model.aggregate([
    $project:{
      expiry: moment("$date_expired").format('h')}]
  ,function(err,res){...});
  </code></pre>
  
  <p>The <code>"$date_expired"</code> is a date in the Schema.</p>
  
  <p>But, when I try to get the results from the above aggregation, I get an error
  <code>FieldPath \'Invalid date\' doesn\'t start with $</code></p>
  
  <p>What could be the error and how do I solve it?</p>