NextDate函数的设计、实现和测试

函数有3个参数：月份、日期和年；它们都具有整数值，且满足以下条件：

1<=月份<=12

1<=日期<=31

1812<=年<=2012

函数返回输入日期之后的那个日期；如果输入无效值，如2000年6月31日，输出指示相应变量超出取值范围；对于无效的日月年组合，如“1999年13月32日”输出“无效输入日期”。

-NextDate function of the design, implementation and testing function has three parameters: month, day and year　they all have integer values, and the following conditions: 1 < = month < = 121 < = date < = 311812 < = years <　= 2012, after the date function returns the input that date　If you enter an invalid value, such as the June 31, 2000, the output indicates the corresponding variable beyond the range　the sun and the moon in an invalid combination, such as " 1999 13 months 32 days"　Output " invalid input date."