code: public void setStart(ReadableInstant start) {
          long startMillis = DateTimeUtils.getInstantMillis(start);
          super.setInterval(startMillis, getEndMillis(), getChronology());
      }
docs: Sets the start of this time interval as an Instant.
  
  @param start  the start of the time interval, null means now
  @throws IllegalArgumentException if the end is before the start