主函数中的调用函数
1 int year = 0,month = 0,day = 0;
2 printf("请输入日期:");
3 scanf("%d/%d/%d",&year,&month,&day);
4 int dayth = dayOfYear(year, month, day);
5 printf("这是一年中的第%d天\n",dayth);
函数声明部分
int dayOfYear(int year,int month,int day);
函数实现部分
1 int dayOfYear(int year,int month,int day){
2 int i = 1,sum = 0;
3 while (i == 1) {
4
5 int a[12] = {31,28,31,30,31,30,31,31,30,31,30,31};
6
7 if (year % 400 == 0 || (year % 100 != 0 && year % 4 == 0)) {
8 a[1]++;
9 }
10
11 if (day > a[month - 1] || day <= 0 || month > 12 || month < 1) {
12 printf("输入日期有误!\n请重新输入:");
13 scanf("%d/%d/%d",&year,&month,&day);
14
15 }else{
16 for (int i = 0; i < month-1; i++) {
17 sum += a[i];
18 }
19 sum += day;
20 i = 0;
21 }
22
23 }
24 return sum;
25 }
原文：http://www.cnblogs.com/yangsea/p/5013632.html