struct tm t;
t.tm_year=4;
t.tm_mon=9;
t.tm_mday=30;
t.tm_hour=0;
t.tm_min=0;
t.tm_sec=0;

printf("%d-%d-%d %d:%d:%dn", t.tm_year, t.tm_mon, t.tm_day, t.tm_hour, t.tm_min, t.tm_sec);
//result: 4-9-30 0:0:0, which means 1904 Oct 30 00:00:00

mktime(&t);

printf("%d-%d-%d %d:%d:%dn", t.tm_year, t.tm_mon, t.tm_day, t.tm_hour, t.tm_min, t.tm_sec);
//result: 4-9-29 23:36:36, which means 1904 Oct 29 23:36:36

#include <time.h>
#include <stdio.h>
int main(void) {
    struct tm t = {
        .tm_year  = 4,
        .tm_mon   = 9,
        .tm_mday  = 30,
        .tm_hour  = 0,
        .tm_min   = 0,
        .tm_sec   = 0,
        .tm_isdst = -1, /* let mktime() figure it out */
    };

    printf("%04d-%02d-%02d %02d:%02d:%02dn",
           1900+t.tm_year, 1+t.tm_mon, t.tm_mday, t.tm_hour, t.tm_min, t.tm_sec);
    // output: 1904-10-30 00:00:00

    mktime(&t);

    printf("%04d-%02d-%02d %02d:%02d:%02dn",
           1900+t.tm_year, 1+t.tm_mon, t.tm_mday, t.tm_hour, t.tm_min, t.tm_sec);
    // output: 1904-10-29 23:36:42
}

$ ./c
1904-10-30 00:00:00
1904-10-30 00:00:00
$ TZ=Asia/Hong_Kong ./c
1904-10-30 00:00:00
1904-10-29 23:36:42
$