code: public DateMidnight toDateMidnight(DateTimeZone zone) {
          Chronology chrono = getChronology().withZone(zone);
          return new DateMidnight(getYear(), getMonthOfYear(), getDayOfMonth(), chrono);
      }
docs: Converts this object to a DateMidnight.
  
  @param zone  the zone to get the DateMidnight in, null means default
  @return the DateMidnight instance