8250636: iso8601_time returns incorrect offset part on MacOS

Backport-of: 1d480a7

src/hotspot/share/runtime/os.cpp

@@ -94,18 +94,6 @@ void os_init_globals() {
   os::init_globals();
 }
 
-static time_t get_timezone(const struct tm* time_struct) {
-#if defined(_ALLBSD_SOURCE)
-  return time_struct->tm_gmtoff;
-#elif defined(_WINDOWS)
-  long zone;
-  _get_timezone(&zone);
-  return static_cast<time_t>(zone);
-#else
-  return timezone;
-#endif
-}
-
 int os::snprintf(char* buf, size_t len, const char* fmt, ...) {
   va_list args;
   va_start(args, fmt);
@@ -158,21 +146,28 @@ char* os::iso8601_time(char* buffer, size_t buffer_length, bool utc) {
       return NULL;
     }
   }
-  const time_t zone = get_timezone(&time_struct);
 
-  // If daylight savings time is in effect,
-  // we are 1 hour East of our time zone
   const time_t seconds_per_minute = 60;
   const time_t minutes_per_hour = 60;
   const time_t seconds_per_hour = seconds_per_minute * minutes_per_hour;
-  time_t UTC_to_local = zone;
-  if (time_struct.tm_isdst > 0) {
-    UTC_to_local = UTC_to_local - seconds_per_hour;
-  }
 
   // No offset when dealing with UTC
-  if (utc) {
-    UTC_to_local = 0;
+  time_t UTC_to_local = 0;
+  if (!utc) {
+#if defined(_WINDOWS)
+    long zone;
+    _get_timezone(&zone);
+    UTC_to_local = static_cast<time_t>(zone);
+
+    // If daylight savings time is in effect,
+    // we are 1 hour East of our time zone
+    if (time_struct.tm_isdst > 0) {
+      UTC_to_local = UTC_to_local - seconds_per_hour;
+    }
+#else
+    // tm_gmtoff already includes adjustment for daylight saving
+    UTC_to_local = -(time_struct.tm_gmtoff);
+#endif
   }
 
   // Compute the time zone offset.