code: public Period plusDays(long daysToAdd) {
          if (daysToAdd == 0) {
              return this;
          }
          return create(years, months, Math.toIntExact(Math.addExact(days, daysToAdd)));
      }
docs: Returns a copy of this period with the specified days added.
  <p>
  This adds the amount to the days unit in a copy of this period.
  The years and months units are unaffected.
  For example, "1 year, 6 months and 3 days" plus 2 days returns "1 year, 6 months and 5 days".
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param daysToAdd  the days to add, positive or negative
  @return a {@code Period} based on this period with the specified days added, not null
  @throws ArithmeticException if numeric overflow occurs