code: public Interval toInterval() {
          DateTimeField field = getField();
          long start = field.roundFloor(getMillis());
          long end = field.add(start, 1);
          Interval interval = new Interval(start, end, getChronology());
          return interval;
      }
docs: Returns the interval that represents the range of the minimum
  and maximum values of this field.
  <p>
  For example, <code>datetime.monthOfYear().toInterval()</code>
  will return an interval over the whole month.
  
  @return the interval of this field
  @since 1.2