code: public static Days ofWeeks(int weeks) {
          if (weeks == 0) {
              return ZERO;
          }
          return new Days(Math.multiplyExact(weeks, DAYS_PER_WEEK));
      }
docs: Obtains a {@code Days} representing the number of days
  equivalent to a number of weeks.
  <p>
  The resulting amount will be day-based, with the number of days
  equal to the number of weeks multiplied by 7.
  
  @param weeks  the number of weeks, positive or negative
  @return the amount with the input weeks converted to days, not null
  @throws ArithmeticException if numeric overflow occurs