code: public DurationProperty setDuration(Duration duration) {
  		DurationProperty prop = (duration == null) ? null : new DurationProperty(duration);
  		setDuration(prop);
  		return prop;
  	}
docs: Sets the length of the pause between alarm repetitions.
  @param duration the duration or null to remove
  @return the property that was created
  @see <a href="http://tools.ietf.org/html/rfc5545#page-99">RFC 5545
  p.99</a>
  @see <a href="http://tools.ietf.org/html/rfc2445#page-94">RFC 2445
  p.94-5</a>