How to resolve “parameter has incomplete type” error?

Question

I\'m a newbie and I need help with debugging my code. When I compile it \'type of formal parameter 1 is incomplete\' and type of formal parameter 2 is incomplete\' error appears

Answer 1

#include 
struct date {
    int month[1];
    int day[1];
    int year[1];

};

int calc_age (struct date birth, struct date current);


int main(void)
{
struct date birth, current;
char c;

printf("Input the birthdate (MM/DD/YY): ");
scanf("%d %c %d %c %d", birth.month, &c, birth.day, &c, birth.year);
printf("Input date to calculate (MM/DD/YY): ");
scanf("%d %c %d %c %d", current.month, &c,  current.day, &c, current.year);

printf("Age is %d years.\n", calc_age(birth, current));

return 0;

}

int calc_age (struct date birth, struct date current) {
int age;

if (birth.month[0] < current.month[0]) {
    age = (current.year[0] - birth.year[0]);
} else if (birth.month[0] > current.month[0]) {
    age = (current.year[0] - birth.year[0] - 1);
} else {
    if (birth.day[0] <= current.day[0]) {
        age = (current.year[0] - birth.year[0]);
    } else {
        age = (current.year[0] - birth.year[0] - 1);
    }
}

return age;
}

You should define the struct before main