title: Convert timestamps stored in a Mongo collection
body: <p>I have various timestamps stored in Mongo collections, some as floats and some as ints. </p>
  
  <p>They are all stored in BST and the server will be switched to UTC soon. How do I convert them within Mongo to be UTC timestamps?</p>
  
  <p>In MySQL I can do this:</p>
  
  <pre><code>UPDATE `table` SET `field` = CONVERT_TZ(`field`, 'Europe/London', 'UTC');
  </code></pre>
  
  <p>Is there a Mongo equivalent?</p>