code: public void add(ReadableDuration duration, int scalar) {
          if (duration != null) {
              add(FieldUtils.safeMultiply(duration.getMillis(), scalar));
          }
      }
docs: Adds a duration to this instant specifying how many times to add.
  <p>
  This will typically change the value of most fields.
  
  @param duration  the duration 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