code: public Period plusMonths(long monthsToAdd) {
          if (monthsToAdd == 0) {
              return this;
          }
          return create(years, Jdk8Methods.safeToInt(Jdk8Methods.safeAdd(months, monthsToAdd)), days);
      }
docs: Returns a copy of this period with the specified months added.
  <p>
  This adds the amount to the months unit in a copy of this period.
  The years and days units are unaffected.
  For example, "1 year, 6 months and 3 days" plus 2 months returns "1 year, 8 months and 3 days".
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param monthsToAdd  the months to add, positive or negative
  @return a {@code Period} based on this period with the specified months added, not null
  @throws ArithmeticException if numeric overflow occurs