lib: support setting process.env.TZ on windows

Fixes: https://github.com/nodejs/node/issues/4230
Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/38642
Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/cli.md

@@ -1669,6 +1669,37 @@ Be aware that unless the child environment is explicitly set, this environment
 variable will be inherited by any child processes, and if they use OpenSSL, it
 may cause them to trust the same CAs as node.
 
+### `TZ`
+<!-- YAML
+added: v0.0.1
+changes:
+  - version:
+     - REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/38642
+    description:
+      Changing the TZ variable using process.env.TZ = changes the timezone
+      on Windows as well.
+  - version:
+     - v13.0.0
+    pr-url: https://github.com/nodejs/node/pull/20026
+    description:
+      Changing the TZ variable using process.env.TZ = changes the timezone
+      on POSIX systems.
+-->
+
+The `TZ` environment variable is used to specify the timezone configuration.
+
+While the Node.js support for `TZ` will not handle all of the various
+[ways that `TZ` is handled in other environments][], it will support basic
+[timezone IDs][] (such as `'Etc/UTC'`, `'Europe/Paris'` or `'America/New_York'`.
+It may support a few other abbreviations or aliases, but these are strongly
+discouraged and not guaranteed.
+
+```console
+$ TZ=Europe/Dublin node -pe "new Date().toString()"
+Wed May 12 2021 20:30:48 GMT+0100 (Irish Standard Time)
+```
+
 ### `UV_THREADPOOL_SIZE=size`
 
 Set the number of threads used in libuv's threadpool to `size` threads.
@@ -1743,3 +1774,5 @@ $ node --max-old-space-size=1536 index.js
 [jitless]: https://v8.dev/blog/jitless
 [libuv threadpool documentation]: https://docs.libuv.org/en/latest/threadpool.html
 [remote code execution]: https://www.owasp.org/index.php/Code_Injection
+[timezone IDs]: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
+[ways that `TZ` is handled in other environments]: https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html

src/node.cc

@@ -937,6 +937,14 @@ int InitializeNodeWithArgs(std::vector<std::string>* argv,
     return 9;
   }
   per_process::metadata.versions.InitializeIntlVersions();
+
+# ifndef __POSIX__
+  std::string tz;
+  if (credentials::SafeGetenv("TZ", &tz) && !tz.empty()) {
+    i18n::SetDefaultTimeZone(tz.c_str());
+  }
+# endif
+
 #endif
 
   NativeModuleEnv::InitializeCodeCache();

src/node_env_var.cc

@@ -2,6 +2,7 @@
 #include "env-inl.h"
 #include "node_errors.h"
 #include "node_external_reference.h"
+#include "node_i18n.h"
 #include "node_process.h"
 
 #include <time.h>  // tzset(), _tzset()
@@ -69,15 +70,32 @@ std::shared_ptr<KVStore> system_environment = std::make_shared<RealEnvStore>();
 }  // namespace per_process
 
 template <typename T>
-void DateTimeConfigurationChangeNotification(Isolate* isolate, const T& key) {
+void DateTimeConfigurationChangeNotification(
+    Isolate* isolate,
+    const T& key,
+    const char* val = nullptr) {
   if (key.length() == 2 && key[0] == 'T' && key[1] == 'Z') {
 #ifdef __POSIX__
     tzset();
+    isolate->DateTimeConfigurationChangeNotification(
+        Isolate::TimeZoneDetection::kRedetect);
 #else
     _tzset();
+
+# if defined(NODE_HAVE_I18N_SUPPORT)
+    isolate->DateTimeConfigurationChangeNotification(
+        Isolate::TimeZoneDetection::kSkip);
+
+    // On windows, the TZ environment is not supported out of the box.
+    // By default, v8 will only be able to detect the system configured
+    // timezone. This supports using the TZ environment variable to set
+    // the default timezone instead.
+    if (val != nullptr) i18n::SetDefaultTimeZone(val);
+# else
+    isolate->DateTimeConfigurationChangeNotification(
+        Isolate::TimeZoneDetection::kRedetect);
+# endif
 #endif
-    auto constexpr time_zone_detection = Isolate::TimeZoneDetection::kRedetect;
-    isolate->DateTimeConfigurationChangeNotification(time_zone_detection);
   }
 }
 
@@ -128,7 +146,7 @@ void RealEnvStore::Set(Isolate* isolate,
   if (key.length() > 0 && key[0] == '=') return;
 #endif
   uv_os_setenv(*key, *val);
-  DateTimeConfigurationChangeNotification(isolate, key);
+  DateTimeConfigurationChangeNotification(isolate, key, *val);
 }
 
 int32_t RealEnvStore::Query(const char* key) const {

src/node_i18n.cc

@@ -542,6 +542,16 @@ bool InitializeICUDirectory(const std::string& path) {
   return status == U_ZERO_ERROR;
 }
 
+void SetDefaultTimeZone(const char* tzid) {
+  size_t tzidlen = strlen(tzid) + 1;
+  UErrorCode status = U_ZERO_ERROR;
+  MaybeStackBuffer<UChar, 256> id(tzidlen);
+  u_charsToUChars(tzid, id.out(), tzidlen);
+  // This is threadsafe:
+  ucal_setDefaultTimeZone(id.out(), &status);
+  CHECK(U_SUCCESS(status));
+}
+
 int32_t ToUnicode(MaybeStackBuffer<char>* buf,
                   const char* input,
                   size_t length) {

src/node_i18n.h

@@ -40,6 +40,8 @@ namespace i18n {
 
 bool InitializeICUDirectory(const std::string& path);
 
+void SetDefaultTimeZone(const char* tzid);
+
 enum idna_mode {
   // Default mode for maximum compatibility.
   IDNA_DEFAULT,

test/parallel/test-datetime-change-notify.js

@@ -0,0 +1,24 @@
+'use strict';
+
+const common = require('../common');
+const { isMainThread } = require('worker_threads');
+
+if (!common.hasIntl)
+  common.skip('Intl not present.');
+
+if (!isMainThread)
+  common.skip('Test not support running within a worker');
+
+const assert = require('assert');
+
+process.env.TZ = 'Etc/UTC';
+assert.match(new Date().toString(), /GMT\+0000/);
+
+process.env.TZ = 'America/New_York';
+assert.match(new Date().toString(), /Eastern (Standard|Daylight) Time/);
+
+process.env.TZ = 'America/Los_Angeles';
+assert.match(new Date().toString(), /Pacific (Standard|Daylight) Time/);
+
+process.env.TZ = 'Europe/Dublin';
+assert.match(new Date().toString(), /Irish/);