3 files changed, 60 insertions, 10 deletions
diff --git a/hash.c b/hash.c
index fc01651f9a..7f0c2be91f 100644
--- a/hash.c
+++ b/hash.c
@@ -5016,7 +5016,7 @@ env_name(volatile VALUE *s)
 static VALUE env_aset(VALUE nm, VALUE val);
 
 static void
-reset_by_modified_env(const char *nam)
+reset_by_modified_env(const char *nam, const char *val)
 {
     /*
      * ENV['TZ'] = nil has a special meaning.
@@ -5025,7 +5025,7 @@ reset_by_modified_env(const char *nam)
      * This hack might works only on Linux glibc.
      */
     if (ENVMATCH(nam, TZ_ENV)) {
-        ruby_reset_timezone();
+        ruby_reset_timezone(val);
     }
 }
 
@@ -5033,7 +5033,7 @@ static VALUE
 env_delete(VALUE name)
 {
     const char *nam = env_name(name);
-    reset_by_modified_env(nam);
+    reset_by_modified_env(nam, NULL);
     VALUE val = getenv_with_lock(nam);
 
     if (!NIL_P(val)) {
@@ -5439,7 +5439,7 @@ env_aset(VALUE nm, VALUE val)
     get_env_ptr(value, val);
 
     ruby_setenv(name, value);
-    reset_by_modified_env(name);
+    reset_by_modified_env(name, value);
     return val;
 }
 
diff --git a/internal/time.h b/internal/time.h
index a3bf0587ec..e21b3574f6 100644
--- a/internal/time.h
+++ b/internal/time.h
@@ -28,7 +28,10 @@ struct timeval rb_time_timeval(VALUE);
 RUBY_SYMBOL_EXPORT_BEGIN
 /* time.c (export) */
 void ruby_reset_leap_second_info(void);
-void ruby_reset_timezone(void);
+#ifdef RBIMPL_ATTR_DEPRECATED_INTERNAL_ONLY
+RBIMPL_ATTR_DEPRECATED_INTERNAL_ONLY()
+#endif
+void ruby_reset_timezone(const char *);
 RUBY_SYMBOL_EXPORT_END
 
 #endif /* INTERNAL_TIME_H */
diff --git a/time.c b/time.c
index d19d1873f8..56f93ec58d 100644
--- a/time.c
+++ b/time.c
@@ -45,6 +45,10 @@
 #include "ruby/util.h"
 #include "timev.h"
 
+#if defined(_WIN32)
+# include "timezoneapi.h" /* DYNAMIC_TIME_ZONE_INFORMATION */
+#endif
+
 #include "builtin.h"
 
 static ID id_submicro, id_nano_num, id_nano_den, id_offset, id_zone;
@@ -703,10 +707,51 @@ static VALUE tm_from_time(VALUE klass, VALUE time);
 
 bool ruby_tz_uptodate_p;
 
+#ifdef _WIN32
+enum {tzkey_max = numberof(((DYNAMIC_TIME_ZONE_INFORMATION *)NULL)->TimeZoneKeyName)};
+static struct {
+    char use_tzkey;
+    char name[tzkey_max * 4 + 1];
+} w32_tz;
+
+static char *
+get_tzname(int dst)
+{
+    if (w32_tz.use_tzkey) {
+        if (w32_tz.name[0]) {
+            return w32_tz.name;
+        }
+        else {
+            /*
+             * Use GetDynamicTimeZoneInformation::TimeZoneKeyName, Windows
+             * time zone ID, which is not localized because it is the key
+             * for "Dynamic DST" keys under the "Time Zones" registry.
+             * Available since Windows Vista and Windows Server 2008.
+             */
+            DYNAMIC_TIME_ZONE_INFORMATION tzi;
+            WCHAR *const wtzkey = tzi.TimeZoneKeyName;
+            DWORD tzret = GetDynamicTimeZoneInformation(&tzi);
+            if (tzret != TIME_ZONE_ID_INVALID && *wtzkey) {
+                int wlen = (int)wcsnlen(wtzkey, tzkey_max);
+                int clen = WideCharToMultiByte(CP_UTF8, 0, wtzkey, wlen,
+                                               w32_tz.name, sizeof(w32_tz.name) - 1,
+                                               NULL, NULL);
+                w32_tz.name[clen] = '\0';
+                return w32_tz.name;
+            }
+        }
+    }
+    return _tzname[_daylight && dst];
+}
+#endif
+
 void
-ruby_reset_timezone(void)
+ruby_reset_timezone(const char *val)
 {
     ruby_tz_uptodate_p = false;
+#ifdef _WIN32
+    w32_tz.use_tzkey = !val || !*val;
+#endif
     ruby_reset_leap_second_info();
 }
 
@@ -1653,11 +1698,9 @@ localtime_with_gmtoff_zone(const time_t *t, struct tm *result, long *gmtoff, VAL
         if (zone) {
 #if defined(HAVE_TM_ZONE)
             *zone = zone_str(tm.tm_zone);
+#elif defined(_WIN32)
+            *zone = zone_str(get_tzname(tm.tm_isdst));
 #elif defined(HAVE_TZNAME) && defined(HAVE_DAYLIGHT)
-# if defined(RUBY_MSVCRT_VERSION) && RUBY_MSVCRT_VERSION >= 140
-#  define tzname _tzname
-#  define daylight _daylight
-# endif
             /* this needs tzset or localtime, instead of localtime_r */
             *zone = zone_str(tzname[daylight && tm.tm_isdst]);
 #else
@@ -5863,6 +5906,10 @@ rb_time_zone_abbreviation(VALUE zone, VALUE time)
 void
 Init_Time(void)
 {
+#ifdef _WIN32
+    ruby_reset_timezone(getenv("TZ"));
+#endif
+
     id_submicro = rb_intern_const("submicro");
     id_nano_num = rb_intern_const("nano_num");
     id_nano_den = rb_intern_const("nano_den");