8288378: [BACKOUT] DST not applying properly with zone id offset set with TZ env variable

Reviewed-by: dholmes

Author: naotoj

src/java.base/unix/native/libjava/TimeZone_md.c

@@ -592,31 +592,18 @@ getGMTOffsetID()
 {
     time_t offset;
     char sign, buf[32];
-    struct tm localtm;
-    time_t clock;
-
-    clock = time(NULL);
-    if (localtime_r(&clock, &localtm) == NULL) {
-        return strdup("GMT");
-    }
-
-    struct tm gmt;
-
-    if (gmtime_r(&clock, &gmt) == NULL) {
-        return strdup("GMT");
-    }
-
-    offset = (localtm.tm_hour - gmt.tm_hour)*3600 + (localtm.tm_min - gmt.tm_min)*60;
+    offset = timezone;
 
     if (offset == 0) {
         return strdup("GMT");
     }
 
+    /* Note that the time offset direction is opposite. */
     if (offset > 0) {
-        sign = '+';
+        sign = '-';
     } else {
         offset = -offset;
-        sign = '-';
+        sign = '+';
     }
     sprintf(buf, (const char *)"GMT%c%02d:%02d",
             sign, (int)(offset/3600), (int)((offset%3600)/60));