Problema con i file (C/C++)

[ghidoz]

Ciao ragazzi, devo scrivere un programma in C che, inserite due date, mi restituisca la differenza in giorni. Inoltre, devo inserire in un file "input.txt" le due date separate da uno spazio e in un file "ouput.txt" il risultato.
Questo è il codice; potreste dirmi cosa sbaglio?

#include <stdio.h>
#include <time.h>
int main ()
{
    static struct tm date1;
    static struct tm date2;
    time_t start, end;
    double diffsec;
    int d, m, y, diffdays, startdate, enddate;
    FILE *in, *out;
    system ("color 1b");
    time (&start);


    in = fopen ("input.txt", "r");
    out = fopen ("output.txt", "r");


    do
    {
        printf ("Enter the start-date using the slash bar (/): ");
        scanf ("%d/%d/%d", &d, &m, &y);
        date1.tm_mday = d;
        date1.tm_mon = m-1;
        date1.tm_year = y - 1900;


        printf ("Enter the end-date using the slash bar (/): ");
        scanf ("%d/%d/%d", &d, &m, &y);
        date2.tm_mday = d;
        date2.tm_mon = m-1;
        date2.tm_year = y - 1900;


    }
    while (mktime(&date2) < mktime(&date1) || d>31 || m>12 || y<1900 || d==0 || m==0);


    fscanf (in, "%.f   %.f", mktime (&date1), mktime (&date2));
    fscanf (out, "%.f", difftime(mktime(&date2), mktime(&date1))/86400);
    fclose (in);
    fclose (out);


    time (&end);
    diffsec = difftime (end, start);


    printf ("\n");
    if (difftime(mktime(&date2), mktime(&date1))/86400 == 1)
        printf ("The difference between the dates is %.f day\n", difftime(mktime(&date2), mktime(&date1))/86400);
    else
        printf ("The difference between the dates is %.f days\n", difftime(mktime(&date2), mktime(&date1))/86400);
    printf ("\n");
    printf ("Time of execution: %f seconds\n", diffsec);
    printf ("\n");
    system ("PAUSE");
}