VS2013编写C程序,为什么可以不加#include<stdio.h>,也不报错呀?程序如下:

C语言小白,请路过的大神门多多指教,程序如下:
main()
{
    int day, month, year, sum, leap;
    printf("\n please input year,month,day \n");
    scanf("%d %d %d", &year, &month, &day);
    switch (month) /*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;
    default:printf("deta 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 && month > 2)/*如果是闰年且月份大于2,总天数应该加上一天*/
        sum++;
    printf("It is the %dth day.\n", sum);
}
希望路过的大神能解答来自小白的疑问,在此抱拳啦!