code: public XmlStringBuilder attribute(String name, Date value) {
          assert value != null;
          return attribute(name, XmppDateTime.formatXEP0082Date(value));
      }
docs: Add a new attribute to this builder, with the {@link java.util.Date} instance as its value,
  which will get formatted with {@link XmppDateTime#formatXEP0082Date(Date)}.
  
  @param name name of attribute
  @param value value of attribute
  @return this XmlStringBuilder