code: public static Date plus(Date self, int days) {
          Calendar calendar = (Calendar) Calendar.getInstance().clone();
          calendar.setTime(self);
          calendar.add(Calendar.DAY_OF_YEAR, days);
          return calendar.getTime();
      }
docs: Add a number of days to this date and returns the new date.
  
  @param self a Date
  @param days the number of days to increase
  @return the new date
  @since 1.0