calendar使用SimpleDateFormat，规定好格式，parser出错即为非法/** * 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;}