设int year,month,day分别表示一个日期中的年月日，试编程求
a)对于任意三个整数，判断其是否为一个合法的日期值。
b)给定一个日期值，计算若干天后的日期值
c)如果有两个日期值，计算他们相距的天数。(set int, year, month, day, respectively, represents a date in the date of the month, try programming
(a) for any three integers, determine whether it is a valid date value.
(b) given a date value, calculates the date value for a few days
C) if there are two date values, calculate the number of days they are apart.)