code: public Completed setCompleted(Date completed) {
  		Completed prop = (completed == null) ? null : new Completed(completed);
  		setCompleted(prop);
  		return prop;
  	}
docs: Sets the date and time that the to-do task was completed.
  @param completed the completion date or null to remove
  @return the property that was created
  @see <a href="http://tools.ietf.org/html/rfc5545#page-94">RFC 5545
  p.94-5</a>
  @see <a href="http://tools.ietf.org/html/rfc2445#page-90">RFC 2445
  p.90-1</a>
  @see <a href="http://www.imc.org/pdi/vcal-10.doc">vCal 1.0 p.29</a>