设int year,month,day分别表示一个日期中的年月日，试编程求

　　对于任意三个整数，判断其是否为一个合法的日期值。

　　给定一个日期值，计算若干天后的日期值

如果有两个日期值，计算他们相距的天数。-Set int./year, respectively, in a date (try programming

For any three integers, decide whether it is a valid date value.

Given a date, calculation of some days after date value

If there are two date value calculation, they are the days.