glibc strptime ignores timezone

bugs.txt

1. strptime isn't using the timezone specifier from "%z" to correct convert non-UTC back to UTC
2. conversion when local tz is UTC or PST to the same value suggests there might be a second bug, depending on implementation.

glibc-strptime.cc

#include <ctype.h>
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <string.h>
	
const time_t correct_t = 1636573045;
const char* s = "Wed, 10 Nov 2021 14:37:25 -0500";
const char* strptime_fmt = "%a, %d %b %Y %H:%M:%S %z";

void testcase() {
	// forcibly zero it between calls
	struct tm *tm;
	time_t t = 0;
	tm = gmtime(&t);

	// s -> tm
	char *remaining_s = strptime(s, strptime_fmt, tm);

	// tm -> t
	t = mktime(tm);

	// explicit that it's a signed value
	long signed int delta_t = (long signed int)(correct_t) - (long signed int)(t);

	if(NULL == remaining_s) { remaining_s = (char*)"(NULL)"; }

	// Used for printing only
	char* tz = getenv("TZ");
	if(!tz) { tz = (char*)"(unset)"; };

	printf("TZ=%s input='%s' correct=%ld parsed=%ld remaining_input='%s' delta=%ld\n", 
			tz,
			s,
			correct_t,
			t,
			remaining_s,
			delta_t
			);
}

int main(int argn, char** argv) {
	unsetenv("TZ");
	testcase();
	setenv("TZ", "UTC", 1);
	testcase();
	setenv("TZ", "EST", 1);
	testcase();
	setenv("TZ", "PST", 1);
	testcase();
}

output.txt

$ g++ glibc-strptime.cc -o glibc-strptime 

$ echo "host '$(date --iso=sec)' '$(date -R)'" ; env -u TZ ./glibc-strptime
host '2021-11-10T11:53:18-08:00' 'Wed, 10 Nov 2021 11:53:18 -0800'
TZ=(unset) input='Wed, 10 Nov 2021 14:37:25 -0500' correct=1636573045 parsed=-1 delta=1636573046
TZ=UTC input='Wed, 10 Nov 2021 14:37:25 -0500' correct=1636573045 parsed=1636555045 delta=18000
TZ=EST input='Wed, 10 Nov 2021 14:37:25 -0500' correct=1636573045 parsed=1636573045 delta=0
TZ=PST input='Wed, 10 Nov 2021 14:37:25 -0500' correct=1636573045 parsed=1636555045 delta=18000

strptime(7)

Manpage extract:

Glibc notes
       For reasons of symmetry, glibc tries to support for  strptime()  the  same
       format  characters  as for strftime(3).  (In most cases, the corresponding
       fields are parsed, but no field in tm is changed.)  This leads to
...
       %z     An RFC-822/ISO 8601 standard timezone specification.

       %Z     The timezone name.