The difftime() function is part of the standard C library and is declared in the <time.h> header file. It calculates the difference between two time values, time1 and time2, in seconds. The function takes two arguments, both of which are of type time_t, which is a type that represents time as the number of seconds elapsed since the epoch (January 1, 1970, 00:00:00 UTC).

The function signature is:

double difftime(time_t time1, time_t time2);

The return value of the difftime() function is a double value representing the difference between the two time values in seconds. The absolute value of the return value is the number of seconds between the two time values, and the sign of the return value indicates the direction of the time difference. If time1 is greater than time2, the return value will be positive. If time2 is greater than time1, the return value will be negative.

Here is an example usage of difftime():

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

int main() {
    time_t start_time, end_time;
    double time_diff;

    // Record the start time
    time(&start_time);

    // Do some work...

    // Record the end time
    time(&end_time);

    // Calculate the time difference
    time_diff = difftime(end_time, start_time);

    printf("The program took %.2f seconds to run.\n", time_diff);

    return 0;
}

In this example, the time() function is used to record the start and end times, and difftime() is used to calculate the difference between them. The resulting time difference is then printed to the console.