code: public List<DateTimePeriod> toYears() {
          ArrayList<DateTimePeriod> list = new ArrayList<DateTimePeriod>();
  
          // default "current" year to start datetime
          DateTime currentStart = getStart();
          // calculate "next" year
          DateTime nextStart = currentStart.plusYears(1);
          // continue adding until we've reached the end
          while (nextStart.isBefore(getEnd()) || nextStart.isEqual(getEnd())) {
              // its okay to add the current
              list.add(new DateTimeYear(currentStart, nextStart));
              // increment both
              currentStart = nextStart;
              nextStart = currentStart.plusYears(1);
          }
  
          return list;
      }
docs: Converts this period to a list of year periods.  Partial years will not be
  included.  For example, a period of "2009-2010" will return a list
  of 2 years - one for 2009 and one for 2010.  On the other hand, a period
  of "January 2009" would return an empty list since partial
  years are not included.
  @return A list of year periods contained within this period