/**
 * Calculates the number of days between two calendar days in a manner
 * which is independent of the Calendar type used.
 *
 * @param d1    The first date.
 * @param d2    The second date.
 *
 * @return      The number of days between the two dates.  Zero is
 *              returned if the dates are the same, one if the dates are
 *              adjacent, etc.  The order of the dates
 *              does not matter, the value returned is always >= 0.
 *              If Calendar types of d1 and d2
 *              are different, the result may not be accurate.
 */
static int getDaysBetween (java.util.Calendar d1, java.util.Calendar d2) {
    if (d1.after(d2)) {  // swap dates so that d1 is start and d2 is end
        java.util.Calendar swap = d1;
        d1 = d2;
        d2 = swap;
    }
    int days = d2.get(java.util.Calendar.DAY_OF_YEAR) -
               d1.get(java.util.Calendar.DAY_OF_YEAR);
    int y2 = d2.get(java.util.Calendar.YEAR);
    if (d1.get(java.util.Calendar.YEAR) != y2) {
        d1 = (java.util.Calendar) d1.clone();
        do {
            days += d1.getActualMaximum(java.util.Calendar.DAY_OF_YEAR);
            d1.add(java.util.Calendar.YEAR, 1);
        } while (d1.get(java.util.Calendar.YEAR) != y2);
    }
    return days;
} // getDaysBetween()