Rewrite the routines day_of_year and month_day with pointers instead of indexing.

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 leap;
    char *p;

    leap = year % 4 == 0 && year%100 != 0 || year %400 == 0;
    p = daytab[leap];
    while(--month)
        day += *++p;
    return day;
}

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

    leap = year %4 == 0 && year%100 != 0 || year%400 == 0;
    p =daytab[leap];
    while(yearday >*++p)
       yearday -= *p;
    *pmonth = p - *(daytab + leap);
    *pday = yearday;
}