Absolute madness with managing time in C

if (row  = mysql_fetch_row(res))
    {
        assert(atoi(row[0]) == contest_id);
        time_t start_time = atoi(row[1]);
        time_t stop_time  = atoi(row[4]);
        int duration = atoi(row[3]);

        struct tm cur_time_tm;
        struct tm start_time_tm;
        struct tm stop_time_tm;

        strncpy(sstart_time, row[1], sizeof(sstart_time));
        strncpy(sstop_time, row[4], sizeof(sstop_time));

        strptime(sstop_time, "%Y-%m-%d %H:%M:%S", &stop_time_tm);
        strptime(sstart_time,  "%Y-%m-%d %H:%M:%S", &start_time_tm);

        printf("ascstarttime=%s;ascstop%s;\n", asctime(&start_time_tm), asctime(&stop_time_tm));
        printf("startstr=%s;stopstr=%s\n", sstart_time, sstop_time);

        start_time = mktime(&start_time_tm);
        stop_time = mktime(&stop_time_tm);

        printf("mkstart=%s;mkstop=%s\n", ctime(&start_time), ctime(&stop_time));
}

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Output:

ascstarttime=Sat Jan  1 04:00:01 2000
;ascstopSat Jan  1 04:00:01 2000
;
startstr=2000-01-01 04:00:01;stopstr=2000-01-01 09:00:01
mkstart=Sat Jan  1 04:00:01 2000
;mkstop=Sat Jan  1 04:00:01 2000

why  asctime(&stop_time_tm)  gives Sat Jan  1 04:00:01 2000
but  sstop_time  outputs                    2000-01-01 09:00:01
?

why sstart_time, sstop_time are different
but start_time_tm, stop_time_tm are always the same.
How?