code: public Interval toInterval(DateTimeZone zone) {
          zone = DateTimeUtils.getZone(zone);
          DateTime start = toLocalDate(1).toDateTimeAtStartOfDay(zone);
          DateTime end = plusMonths(1).toLocalDate(1).toDateTimeAtStartOfDay(zone);
          return new Interval(start, end);
      }
docs: Converts this object to an Interval representing the whole month.
  <p>
  The interval will use the chronology of the year-month in the specified zone.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param zone  the zone to get the Interval in, null means default
  @return an interval over the month, never null