程序代码：

#include "stdio.h"

void main()
{
        int day,mouth,year,sum,leap;
        printf("\nplease input year,mouth,day\n");
        scanf("%d,%d,%d",&year,&mouth,&day);
        switch (mouth) /*先计算某月以前月份的总天数*/
        {
            case 1:sum=0;break;
            case 2:sum=31;break;
            case 3:sum=59;break;
            case 4:sum=90;break;
            case 5:sum=120;break;
            case 6:sum=151;break;
            case 7:sum=181;break;
            case 8:sum=212;break;
            case 9:sum=243;break;
            case 10:sum=273;break;
            case 11:sum=304;break;
            case 12:sum=334;break;
            defaultrintf("data error");break;
        }
        sum=sum+day; /* 再加上某天的天数*
        if (year%400==0||(year%4==0&&year%100!=0))/*判断是不是闰年*/
        leap=1;
        else
        leap=0;
        if (leap==1&&mouth>2)/*如果是闰年且月份大于2，总数天数应该加1*/
        sum++;
        printf ("It is the %dth day.",sum);
}

程序代码：

# include <stdio.h>

#define LEAPYEAR 0
#define PINGNIAN 1

int
main()
{
    int date_array[2][13] =
    { { 31, 29 , 31 ,30 ,31 ,30 , 31 , 31 , 30 , 31 , 30 , 31 } ,
    { 31, 28 , 31 ,30 ,31 ,30 , 31 , 31 , 30 , 31 , 30 , 31} } ;
    int year , month , day ;
    int i ,sum_of_day = 0 ;
    scanf( "%d%d%d" , &year , & month , &day ) ;
    if( year%400 == 0 || ( year%4 == 0 && year % 100 != 0 ) )
    {
        for( i = 0 ; i < month - 1 ; i++ )
            sum_of_day += date_array[LEAPYEAR][i] ;
        sum_of_day += day ;
    }
    else
    {
        for( i = 0 ; i < month - 1 ; i++ )
            sum_of_day += date_array[PINGNIAN][i] ;
        sum_of_day += day ;
    }
    printf( "The date is %d\n" , sum_of_day ) ;
    return 0 ;
}