Exercise 5-8: Date conversion with error checking

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;
    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;

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

Each input line holds a year, month and day, or a year and a day of the year; years are positive. For the first, print day_of_year's result; for the second, print the month and day month_day finds, separated by a blank. Print error instead for a month, day or day of the year that doesn't exist in that year.