title: Creating a GregorianCalendar instance from milliseconds
body: <p>I have a certain time in milliseconds (in a <code>Timestamp</code> object) and I want to use it to create a <code>GregorianCalendar</code> object. How can I do that?</p>
  
  <p><strong>EDIT</strong>: How do I do the reverse?</p>