code: public static DateValue add(DateValue date, DateValue duration) {
  		DTBuilder db = new DTBuilder(date);
  		db.year += duration.year();
  		db.month += duration.month();
  		db.day += duration.day();
  		if (duration instanceof TimeValue) {
  			TimeValue tdur = (TimeValue) duration;
  			db.hour += tdur.hour();
  			db.minute += tdur.minute();
  			db.second += tdur.second();
  			return db.toDateTime();
  		}
  		return (date instanceof TimeValue) ? db.toDateTime() : db.toDate();
  	}
docs: Adds a duration to a date.
  @param date the date
  @param duration the duration to add to the date
  @return the result