code: public void add(DurationFieldType type, int amount) {
          if (type == null) {
              throw new IllegalArgumentException("Field must not be null");
          }
          if (amount != 0) {
              setMillis(type.getField(getChronology()).add(getMillis(), amount));
          }
      }
docs: Adds to the instant specifying the duration and multiple to add.
  
  @param type  a field type, usually obtained from DateTimeFieldType, not null
  @param amount  the amount to add of this duration
  @throws IllegalArgumentException if the value is null or invalid
  @throws ArithmeticException if the result exceeds the capacity of the instant