[date] Refactor TimezoneCache to be separate from the OS

src/base/platform/platform-posix.cc

Index: src/base/platform/platform-posix.cc
diff --git a/src/base/platform/platform-posix.cc b/src/base/platform/platform-posix.cc
index f195649e5e9f818c7bfab97f67f3439abbf26061..3b8045a7675d7736f64f473629ddf14e99e04808 100644
--- a/src/base/platform/platform-posix.cc
+++ b/src/base/platform/platform-posix.cc
@@ -37,6 +37,8 @@
 #include <cmath>
 #include <cstdlib>
 
+#include "src/base/platform/platform-posix.h"
+
 #include "src/base/lazy-instance.h"
 #include "src/base/macros.h"
 #include "src/base/platform/platform.h"
@@ -380,26 +382,25 @@ double OS::TimeCurrentMillis() {
   return Time::Now().ToJsTime();
 }
 
-
-class TimezoneCache {};
-
-
-TimezoneCache* OS::CreateTimezoneCache() {
-  return NULL;
-}
-
-
-void OS::DisposeTimezoneCache(TimezoneCache* cache) {
-  DCHECK(cache == NULL);
+const char* PosixTimezoneCache::LocalTimezone(double time) {
+  if (std::isnan(time)) return "";
+  time_t tv = static_cast<time_t>(std::floor(time / msPerSecond));
+  struct tm tm;
+  struct tm* t = localtime_r(&tv, &tm);
+  if (!t || !t->tm_zone) return "";
+  return t->tm_zone;
 }
 
-
-void OS::ClearTimezoneCache(TimezoneCache* cache) {
-  DCHECK(cache == NULL);
+double PosixTimezoneCache::LocalTimeOffset() {
+  time_t tv = time(NULL);
+  struct tm tm;
+  struct tm* t = localtime_r(&tv, &tm);
+  // tm_gmtoff includes any daylight savings offset, so subtract it.
+  return static_cast<double>(t->tm_gmtoff * msPerSecond -
+                             (t->tm_isdst > 0 ? 3600 * msPerSecond : 0));
 }
 
-
-double OS::DaylightSavingsOffset(double time, TimezoneCache*) {
+double PosixTimezoneCache::DaylightSavingsOffset(double time) {
   if (std::isnan(time)) return std::numeric_limits<double>::quiet_NaN();
   time_t tv = static_cast<time_t>(std::floor(time/msPerSecond));
   struct tm tm;