difftime() return different data type in c on vs2022

I found a weird thing in my program. Here are two examples

#include <stdio.h>

int main() {

    time_t t1, t2;
    t1 = 1735689614;
    t2 = 1735689600;

    printf("difftime =%d \n", difftime(t1, t2)); // difftime() return int
}

#include <stdio.h>
#include <time.h>

int main() {

    time_t t1, t2;
    t1 = 1735689614.0;
    t2 = 1735689600.0;

    printf("difftime =%f \n", difftime(t1, t2)); // difftime() return double 
}

you may notice that #include<time.h> and the return value of difftime()

The difftime() of two examples are different functions.

I know the double difftime() is the standard function of time.h (C11). But where the int difftime() come from?

It is a very weird and subtle problem when c programming in vs2022. I have change the c standard to C11 in vs2022. And The program compiled OK. I thought it would use the double difftime() without #inlcude<time.h>.

While this problem can be clearly seen when using gcc

implicit declaration of function 'difftime' [-Wimplicit-function-declaration]

double difftime() int difftime()

In the first version, you don't include <time.h>, and therefore difftime is implicitly declared as returning int whereas it actually returns a double.

BTW the compiler warns you with this message: warning C4013: 'difftime' undefined; assuming extern returning int.

So basically the first version of your code is broken.

Consider all warnings containing the words "undefined" and "assuming" as errors.