程序代码：

#include<stdio.h>

static char daytab[2][13] =
{
    { 0, 31, 28 , 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 },
    { 0, 31, 29 , 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }
};

int main()
{
    int day_of_year(int year, int month, int day);
    void month_day(int year1, int day1, int *pmonth, int *pday);
    int year, month, day;
    int year1, day1, yearmonth, yearday;

    printf("Please input year, month, day converted to year.day!\n");
    scanf("%d%d%d", &year, &month, &day);
    printf("This day is year : %d days : %d\n", year, day_of_year(year, month, day));
    printf("\nPlease input year, day converted to month , day\n");
    scanf("%d%d", &year1, &day1);
    month_day(year1, day1, &yearmonth, &yearday);
    printf("This times is : %d %d %d", year1, yearmonth, yearday);
    return 0;
}

int day_of_year(int year, int month, int day)
{
    int i, leap;

    leap = !(year % 4) && year % 100 || !(year % 400);
    for (i = 1; i < month; ++i)
        day += daytab[leap][i];
    return day;
}

void month_day(int year1, int day1, int *pmonth, int *pday)
{
    int i, leap;
    
    leap = !(year1 % 4) && year1 % 100 || !(year1 % 400);
    for (i = 1; daytab[leap][i] < day1; ++i)
        day1 -= daytab[leap][i];
    *pmonth = i;
    *pday = day1;
}