C :

#include<stdio.h>
struct datetime
{int year;
int month;
int day;
};
int days(struct datetime date);
int main()
{
int count_day;
struct datetime date;
printf("请输入日期\n");
scanf("%d %d %d",&date.year,&date.month,&date.day);
count_day=days(date);
printf("日期%d/%d/%d是%d年的第%d天\n",date.year,date.month,date.day,date.year,count_day);
return 0;	
}
int days(struct datetime date)
{
	int result=0;
	int year=date.year,month=date.month,day=date.day;
	 if(month>1)
	 {
	switch(month-1)
	{
	case 12:
		result+=31;
		case 11:
			result+=30;
				case 10:
		result+=31;
			case 9:
		result+=30;
			case 8:
		result+=31;
			case 7:
		result+=31;
			case 6:
		result+=30;
			case 5:
		result+=31;
			case 4:
		result+=30;
			case 3:
		result+=31;
			case 2:
		{
		if(year%400==0||year%100!=0&&year%4==0)result+=29;
		else 
		result+=28;	
		}
		case 1:
			result+=31;
		}
			result+=day;
		}
		else 
	result=day;
	return result ;					
}