class ExtendedChronology extends AssembledChronology {

    public static DateTime withPeriodStart(DateTime dt) {
        int div = (dt.getMonthOfYear() % 3);
        int d = dt.getMonthOfYear() - Math.max(0, ((div == 0 ? 3 : div) - 1));
        dt = dt.withMonthOfYear(d);
        long millis = PeriodType.QUARTER.getComparator().getLowerLimit().getField(
        	getInstance()).roundFloor(dt.getMillis());
    	return new DateTime(millis);
    }

    private  ExtendedChronology(Chronology base);

}