code: public void add(ReadablePeriod period, int scalar) {
          if (period != null) {
              setMillis(getChronology().add(period, getMillis(), scalar));  // set via this class not super
          }
      }
docs: Adds a period to this instant specifying how many times to add.
  <p>
  This will typically change the value of most fields.
  
  @param period  the period to add, null means add zero
  @param scalar  direction and amount to add, which may be negative
  @throws ArithmeticException if the result exceeds the capacity of the instant