C语言小程序（二）、计算第二天日期

输入一个日期，判断下一个日期是哪一天，如果日期输入错误，则调用Linux下的cal命令显示输入的月份。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int year, month, day;
const int day_30 = 30;
const int day_31 = 31;
int day_month_2 = 0;
char err_flag = 0;

void compute()
{
	printf("输入年月日(1992-7-19)：");
	scanf("%d-%d-%d",&year, &month, &day);
	if((year%4==0 && year%100!=0) || (year%400==0))
	{
		day_month_2 = 29;
	}
	else
	{
		day_month_2 = 28;
	}
	if(month>=1 && month<=12)
	{
		if(month == 2)
		{
			if(day > day_month_2)
			{
				err_flag = 1;
			}
			else if(day == day_month_2)
			{
				month++;
				day = 1;
			}
			else
			{
				day++;
			}
		}
		else if(month==1 || month==3 || month==5 || month==7 ||\
				month==8 || month==10 || month==12)
		{
			if(day > day_31)
			{
				err_flag = 1;
			}
			else if(day == day_31)
			{
				month++;
				day = 1;
			}
			else
			{
				day++;
			}
		}
		else
		{
			if(day > day_30)
			{
				err_flag = 1;
			}
			else if(day == day_30)
			{
				month++;
				day = 1;
			}
			else
			{
				day++;
			}
		}
		if(month > 12)
		{
			year++;
			month = 1;
		}
	}
	else
	{
		err_flag = 1;
	}
}

int main()
{
	time_t now;
	char ch, c;
	char call_cal[30];
	do
	{
		compute();
		if(err_flag)
		{
			err_flag = 0;
			printf("日期输入错误，调用cal命令查询%d年%d月：\n\n",year, month);
			sprintf(call_cal, "/usr/bin/cal %d %d",month, year);
			system(call_cal);
			//system("clear");
			while((c=getchar())!='\n' && (c=getchar())!=EOF);
			printf("是否重新输入(y/n)：");
		}
		else
		{
			printf("第二天为%d-%d-%d。\n",year, month, day);
			while((c=getchar())!='\n' && (c=getchar())!=EOF);
			printf("是否再次输入日期(y/n)：");
		}
		ch = getchar();
	}
	while(ch=='y' || ch=='Y' || ch=='\n');
	time(&now);
	printf("Now is %s",ctime(&now));
	puts("Having a goodday!");

	return 0;
}