Aix: Fix UTC time offset calculation with --no-icu-timezone-data flag

Change-Id: I3b504d7d22da475b317f5877bc0a5a642017754f
Reviewed-on: https://chromium-review.googlesource.com/c/1363531Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#58109}

src/base/platform/platform-aix.cc

@@ -36,6 +36,20 @@ namespace v8 {
 namespace base {
 
 
+int64_t get_gmt_offset(const tm& localtm) {
+  // replacement for tm->tm_gmtoff field in glibc
+  // returns seconds east of UTC, taking DST into account
+  struct timeval tv;
+  struct timezone tz;
+  int ret_code = gettimeofday(&tv, &tz);
+  // 0 = success, -1 = failure
+  DCHECK_NE(ret_code, -1);
+  if (ret_code == -1) {
+    return 0;
+  }
+  return (-tz.tz_minuteswest * 60) + (localtm.tm_isdst > 0 ? 3600 : 0);
+}
+
 class AIXTimezoneCache : public PosixTimezoneCache {
   const char* LocalTimezone(double time) override;
 
@@ -54,13 +68,15 @@ const char* AIXTimezoneCache::LocalTimezone(double time_ms) {
 }
 
 double AIXTimezoneCache::LocalTimeOffset(double time_ms, bool is_utc) {
-  // On AIX, struct tm does not contain a tm_gmtoff field.
+  // On AIX, struct tm does not contain a tm_gmtoff field, use get_gmt_offset
+  // helper function
   time_t utc = time(nullptr);
   DCHECK_NE(utc, -1);
   struct tm tm;
   struct tm* loc = localtime_r(&utc, &tm);
   DCHECK_NOT_NULL(loc);
-  return static_cast<double>((mktime(loc) - utc) * msPerSecond);
+  return static_cast<double>(get_gmt_offset(*loc) * msPerSecond -
+                             (loc->tm_isdst > 0 ? 3600 * msPerSecond : 0));
 }
 
 TimezoneCache* OS::CreateTimezoneCache() { return new AIXTimezoneCache(); }