关于形参与实参不兼容的问题（C语言）

Question

sum(date)处显示实参与形参不兼容，请问是为什么

#include<stdio.h>
struct y_m_d
{
    int year;
    int month;
    int day;
}date;
int main()
{
    int sum(struct y_m_d data);
    printf("input year month day:");
    scanf("%d%d%d", &date.year, &date.month, &date.day);
    printf("%d %d is the %dth day in %d.\n", date.month, date.day, sum(date), date.year);
}

int sum(struct y_m_d data)
{
    int days;
    switch (data.month)
    {
    case 1:days = data.day; break;
    case 2:days = data.day + 31; break;
    case 3:days = data.day + 59; break;
    case 4:days = data.day + 90; break;
    case 5:days = data.day + 120; break;
    case 6:days = data.day + 151; break;
    case 7:days = data.day + 181; break;
    case 8:days = data.day + 212; break;
    case 9:days = data.day + 243; break;
    case 10:days = data.day + 273; break;
    case 11:days = data.day + 304; break;
    case 12:days = data.day + 334; break;
    default:
        break;
    }
    if (data.year % 4 == 0 && data.year % 100 != 0 || data.year % 400 == 0 && data.month >= 3)
        days += 1;
    return sum;
}

Answer 1

return sum这一句，不应该是return days吗？