public Date rollDate(Date base, TimeZone zone) {
        // as far as I know, the Locale for the calendar only affects week
    	// number calculations, and since DateTickUnit doesn't do week
    	// arithmetic, the default locale (whatever it is) should be fine
    	// here...
        Calendar calendar = Calendar.getInstance(zone);
        calendar.setTime(base);
        calendar.add(getCalendarField(this.rollUnit), this.rollCount);
        return calendar.getTime();