code: public static String convertDateToDateString(Date date) {
  		if (date == null) {
  			return null;
  		} else {
  			// DateFormat df = new SimpleDateFormat("yyyy-MM-dd'Z'");
  			DateTimeFormatter df = FORMATTER_DATE_Z;
  			return df.print(date.getTime());
  		}
  	}
docs: Converts an instance of java.util.Date into a String using the date
  format: yyyy-MM-ddZ.
  
  @param date
  Instance of java.util.Date.
  @return Corresponding date string (returns null if Date argument is
  null).