code: public Interval withStart(ReadableInstant start) {
          long startMillis = DateTimeUtils.getInstantMillis(start);
          return withStartMillis(startMillis);
      }
docs: Creates a new interval with the specified start instant.
  
  @param start  the start instant for the new interval, null means now
  @return an interval with the end from this interval and the specified start
  @throws IllegalArgumentException if the resulting interval has end before start