参考方法：

#include<stdio.h>
int main()
{
    printf("请输入日期（格式：xxxx.xx.xx）：");
    int year,month,date;
    scanf("%d.%d.%d",&year,&month,&date);
    int md[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};/*多写一个，方便标号*/
    /*判断是否是闰年*/
    if(((year%4==0)&&(year%100!=0))||(year%400==0)) {
        md[2]+=1;
    }
    /*判断日期符合实际*/
    if(year<=0||month<1||month>12||date<1||date>md[month]) {
        printf("输入数据无效！\n");
        return 0;
    }
    int mi,days=0;
    for (mi=0;mi<month;mi++) {
        days+=md[mi];
    }
    printf("%d.%d.%d是这一年的第%d天。\n",year,month,date,days+date);
    return 0;
}

参考方法:

#include <stdio.h>

int main(){

    int year,month,day;
     // 一、 二、 三、 四、 五、六 、七、八、九、十、十一、十二
    int monthOfDayNumber[] = {31, 28, 31, 30, 31, 30,31,31,30,31,30,31};
    printf("\n请输入年、月、日，格式为：年,月,日（2015,12,10）\n");
    scanf("%d,%d,%d",&year,&month,&day);  // 格式为：2015,12,10
    int sum=0,leap=0;
    if(year%400==0||(year%4==0&&year%100!=0)){
        leap = 1;
    }

    if(month>=3){
        printf("闰年： %d\n",leap);
        sum = sum + leap;
    }

    for(;month-2>=0;month--){
        sum += monthOfDayNumber[month-2];
    }
    sum += day;
    printf("此日期是%d年的第%d天\n",year,sum);
    return 0;
}

参考方法：

#include<stdio.h>

//某月的天数 
int getMonthDay(int month)
{
    int day = 0;
    switch(month){
        case 2:
            day = 28;
            break; 
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            day = 31;
            break;
        case 4:
        case 6:
        case 9:
        case 11:
            day = 30;
            break;        
    }
    return day;
}

//是否闰年
int isleap(int year)
{
    if ( (year%4==0 && year%100!=0) || (year%400==0) ){
        return 1;
    }
    return 0;
}

int main()
{
    int year, month, day;
    printf("请输入年月日(2019-5-1)：\n");
    scanf("%d-%d-%d", &year, &month, &day);
    
    int days = day, m;
    for(m=1; m<month; m++){
        days += getMonthDay(m);
    }
    if (isleap(year)==1){
        days += 1;
    }
    printf("%d-%d-%d是这一年的第%d天", year, month, day, days);
    return 0;
}