title: new Date() in aggregate $project
body: <p>For one of my collections, which must remain unix timestamp instead of isodate, I usually convert the timestamp with new Date(unix_timestamp).</p>
  <p>Now I need the <code>new Date(ts)</code> in an aggregation. (the example is in PHP)</p>
  <blockquote>
  <p>'$project' =&gt; array ( 'day' =&gt; '$new Date(ts)', ...</p>
  <p>'$group' =&gt; array ( &quot;_id&quot; =&gt; array('day' =&gt; '$day)', ...),...</p>
  </blockquote>
  <p>does not arrive in the results. The field &quot;day&quot; in the result is missing entirely.</p>
  <p>How to get this converted within the aggregation?</p>