There is no error checking in day_of_year or month_day.Remedy this defect.

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}
};

/*day_of_year: set day of year from month & day    */
int day_of_year(int year, int month, int day){
    int i, leap;

    leap =year %4 == 0 && year %100 != 0 || year%400 == 0);
    if(month < 1 || month > 12)
         return -1;
    if(day < 1 || day > daytab[leap][month])
       return -1;
    for(i =1; i < month; i++)
       day +=daytab[leap][i];
    return day;
}

/*month_day: set month, day from day of year    */
void month_day(int year, int yearday, int *pmonth, int *pday){
     int i, leap;

     if (year < 1){
        *pmonth = -1;
        *pday = -1;
        return ;
     }
     leap = year % 4 == 0 && year %100 != 0 || year %400 == 0;
     for(i = 1; i <=12 && yearday > daytab[leap][i];i++)
          yearday -=daytab[leap][i];
    if (i > 12 && yearday > daytab[leap][12]){
       *pmonth = -1;
       *pday = -1;
    }else{
       *pmonth = i;
       *pday = yearday;
    }
}