title: Update MongoDB date field using value of another date field
body: <p>I have a collection with the following instance:</p>
  <p><code>{ id: 1, createdAt: ISODate(&quot;2018-12-25T21:18:06.791+0000&quot;) }</code></p>
  <p>And I would like to use an update operation for adding another date field that take its value from the <code>createdAt</code> field:</p>
  <p><code>{ id: 1, createdAt: ISODate(&quot;2018-12-25T21:18:06.791+0000&quot;), newField: ISODate(&quot;2018-12-25T21:18:06.791+0000&quot;) }</code></p>
  <p>Is there a possible way of doing so? I tried this code right here:</p>
  <pre><code>collection.updateMany({
      {},
      $set: {
          newField: '$createdAt'
      }
  });
  </code></pre>
  <p>But it failed to generate my desire result.
  Thanks!</p>
  <p>(Mongodb version 4.0.2)</p>