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;
}