code: public Interval toInterval() {
          Chronology chrono = getChronology();
          long start = getMillis();
          long end = DurationFieldType.days().getField(chrono).add(start, 1);
          return new Interval(start, end, chrono);
      }
docs: Converts this object to an <code>Interval</code> encompassing
  the whole of this day.
  <p>
  The interval starts at midnight 00:00 and ends at 00:00 the following day,
  (which is not included in the interval, as intervals are half-open).
  
  @return an interval over the day