composefs-from-json: Store mtimes in UTC

The time as stored in json includes a timzeone, but for the
erofs value we want to store the time in UTC. Unfortunately the
time apis are kind of lame here, timegm() always assumes the
data in the struct tm is in gmt time. So we have to extract the
gmt offset from the initial strptime call and apply it manually.

Signed-off-by: Alexander Larsson <alexl@redhat.com>

tools/composefs-from-json.c

@@ -385,10 +385,16 @@ static struct lcfs_node_s *get_node(struct lcfs_node_s *root, const char *what)
 static void parse_time(const char *time, struct timespec *ts)
 {
 	struct tm tm;
+	time_t t;
+	long gmtoff;
 
+	memset(&tm, 0, sizeof(tm));
 	strptime(time, "%Y-%m-%dT%H:%M:%S%z", &tm);
+	gmtoff = tm.tm_gmtoff;
 
-	ts->tv_sec = mktime(&tm);
+	tm.tm_isdst = -1;
+	t = timegm(&tm);
+	ts->tv_sec = t - gmtoff;
 	ts->tv_nsec = 0;
 }
 
@@ -667,6 +673,8 @@ int main(int argc, char **argv)
 	cleanup_free FILE **input_files = NULL;
 	bool no_sandbox = false;
 
+	tzset();
+
 	while ((opt = getopt_long(argc, argv, ":CR", longopts, NULL)) != -1) {
 		switch (opt) {
 		case OPT_OUT: