How to get GMT offset of given Timezone.

    Calculating GMT offset of any timezone is a little bit tricky. I need to deal with this situation, in one of my project. I have written a small program.
    I have considered /usr/share/zoneinfo as default timezone directory. You need to change if you timezone files are located any where else. Your feedback are valuable for me.

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>
#include <unistd.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>

#define ABS(ent) (ent) = ((ent)>0?(ent):0-(ent))
#define TZ_DIR  "/usr/share/zoneinfo"

static void
display_gmt_off(long tz)
{
 int32_t hrs, min;

 hrs = tz/3600;
 min = tz % 3600;
 min = min/60;
 ABS(min);
 ABS(hrs);

 fprintf(stdout, "(GMT");
 if (tz < 0) {
  fprintf(stdout, "+");
 } else if (tz > 0) {
  fprintf(stdout, "-");
 }
 if (hrs > 0 || min > 0) {
  fprintf(stdout, "%d:%d", hrs, min);
 }
 fprintf(stdout, ")\n");
}

static int
invalid_timezone(const char *tz)
{
 char tzpath[1024];
 struct stat st;

 snprintf(tzpath, sizeof(tzpath), "%s/%s", TZ_DIR, tz);
 memset(&st, 0, sizeof(st));
 return stat(tzpath, &st);
}

int
main(int argc, char **argv)
{
 const char *tz;

 if (argc != 2) {
  fprintf(stderr, "Usage %s \n", basename(argv[0]));
  return 1;
 }
 tz = argv[1];

 if (invalid_timezone(tz)) {
  fprintf(stderr, "Invalid timezone %s\n", tz);
  return 1;
 }
 if (setenv("TZ", tz, 1) < 0) {
  fprintf(stderr, "Cannot set TZ(%s)\n", tz);
  return 1;
 }
 tzset();
 display_gmt_off(timezone); /* extern variable in time.h */
 return 0;
}