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