code: public Interval toInterval(DateTimeZone zone) {
          zone = DateTimeUtils.getZone(zone);
          DateTime start = toDateTimeAtStartOfDay(zone);
          DateTime end = plusDays(1).toDateTimeAtStartOfDay(zone);
          return new Interval(start, end);
      }
docs: Converts this object to an Interval representing the whole day.
  <p>
  The interval may have more or less than 24 hours if this is a daylight
  savings cutover date.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param zone  the zone to get the Interval in, null means default
  @return a interval over the day