Converts the number in the/proc/uptime file to long and then to the date

Ideas:
1, formatted to read the two number of files, saved in two strings.
2,/proc/uptime in seconds, discard decimal. Converts the integer part to long.
3, from long to date.

#include <stdio.h>

int main ()
{
    FILE *fp = NULL;
    fp = fopen ("uptime.c", "R");
    if (fp = = NULL)
    {
        printf ("Open File failed!\n");
        return 0;
    }
    Save two numbers, the length is slightly larger than the number of files
    //You can use Cat/proc/uptime in advance to see
    char t[20];
    Char u[20];
    FSCANF (FP, "%s", T, u);

    Count the number of integer digits of the first number, and turn to long, omitting the 2nd number processing
   unsigned long dt = 0;
    int i;
    For (i=0 i<20; i++)
    {
        if (t[i] = = '. ')
            break;
    int j = 1;
    For (i--i>=0; i--)
    {
        dt + = (t[i]-' 0 ') * j;
        J *=;

    Long turns to date and outputs
    unsigned long day, hour, minute, second;
    Day = dt/(60*60*24);
    DT = dt% (60*60*24);
    hour = DT/(60*60);
    DT = dt% (60*60);
    minute = DT/60;
    second = dt%;

    printf ("%ld:%ld:%ld:%ld\n", day, hour, minute, second);

    Fclose (FP);
    return 0;
}