Empty `TZ` environment should use UTC, not local time

[drinkcat]

At least on Linux (glibc at least, musl as well AFAICT), TZ= forces UTC timezone, not the local timezone. That doesn't seem to be documented anywhere though...

Example C code:

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

int main(int argc, char *argv[]) {
    char outstr[200];
    time_t t;
    struct tm *tmp;

    t = time(NULL);
    tmp = localtime(&t);
    strftime(outstr, sizeof(outstr), "%c %Z", tmp);
    puts(outstr);
}

Example output. Setting TZ= is equivalent to setting TZ=UTC:

 $ ./test
Sat Apr  5 14:53:55 2025 CEST << that's /etc/localtime timezone
$ TZ= ./test
Sat Apr  5 12:53:57 2025 UTC
$ TZ=UTC ./test
Sat Apr  5 12:54:05 2025 UTC

Rust test program:

use chrono::Local;

fn main() {
    println!("{}", Local::now());
}

$ cargo run
2025-04-05 14:58:04.069014727 +02:00
$ TZ= cargo run
2025-04-05 14:58:06.276775336 +02:00 <<< This should probably be +02:00
$ TZ=UTC cargo run
2025-04-05 12:58:09.002152546 +00:00

This causes issues in uutils/coreutils: uutils/coreutils#7498

[djc]

Thanks for the report! Would you be able to submit a PR?