code: public Months plus(int months) {
          if (months == 0) {
              return this;
          }
          return Months.months(FieldUtils.safeAdd(getValue(), months));
      }
docs: Returns a new instance with the specified number of months added.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param months  the amount of months to add, may be negative
  @return the new period plus the specified number of months
  @throws ArithmeticException if the result overflows an int