code: public static Date addDays(Date date, int numberOfDays) {
      return Date.from(date.toInstant().plus(numberOfDays, ChronoUnit.DAYS));
    }
docs: Adds a number of days to a date returning a new object.
  The original date object is unchanged.
  
  @param date         the date, not null
  @param numberOfDays the amount to add, may be negative
  @return the new date object with the amount added