title: How i manipulate two dates and update the third?
body: <p>How I can subtract the field <code>todaytime</code> and the field <code>Timestamp</code> and add the result in days for 1 million objects in a collection? I need that for all the objects using Mongo shell.</p>
  
  <p>My collection contains object like the next one:</p>
  
  <pre><code>{ "_id" : ObjectId("5504cb8edd5af617ca9ca192"),
  
  "Timestamp" : ISODate("2014-04-06T18:44:58.314Z"),
  
  "todaytime" : ISODate("2014-04-02T01:11:18.965Z")}
  </code></pre>