title: Use the value of a queried field in $set on Mongo
body: <p>I need to convert all non-date types (ISO strings, unix timestamps...) to js Date objects. So is there a way to modify the value of a field natively in Mongo? I'm thinking something along the lines of <code>$inc</code> - as it modifies the current value in place.</p>
  
  <p>For example: given the following doc structure in the "projects" collection:</p>
  
  <pre><code>{
    _id: '123456',
    created: '1449224127049',
    name: 'My document',
    ...
  }
  </code></pre>
  
  <p>Is there a way to do something like:</p>
  
  <pre><code>db.projects.update(
    {created: {$not: {$type: 9} } },
    {$set: {created: new Date( ??? ) } },
    {multi: true}
  );
  </code></pre>