LibJS+LibUnicode: Change time zones in a way that works on Windows

On Windows, ICU does not look at the TZ environment variable at all. So
to support changing time zones in test-js, let's set ICU's default time
zone directly.

Note that we no longer deal with "null" time zone strings. We just cache
whatever ICU thinks is the current time zone before attempting to change
it, for which we never have a null result.

Co-authored-by: Andrew Kaster <andrew@ladybird.org>

Libraries/LibUnicode/TimeZone.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2024-2025, Tim Flynn <trflynn89@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -24,8 +24,8 @@ String current_time_zone()
 
     UErrorCode status = U_ZERO_ERROR;
 
-    auto time_zone = adopt_own_if_nonnull(icu::TimeZone::detectHostTimeZone());
-    if (!time_zone)
+    auto time_zone = adopt_own_if_nonnull(icu::TimeZone::createDefault());
+    if (!time_zone || *time_zone == icu::TimeZone::getUnknown())
         return "UTC"_string;
 
     icu::UnicodeString time_zone_id;
@@ -46,6 +46,18 @@ void clear_system_time_zone_cache()
     cached_system_time_zone.clear();
 }
 
+ErrorOr<void> set_current_time_zone(StringView time_zone)
+{
+    auto time_zone_data = TimeZoneData::for_time_zone(time_zone);
+    if (!time_zone_data.has_value())
+        return Error::from_string_literal("Unable to find the provided time zone");
+
+    icu::TimeZone::setDefault(time_zone_data->time_zone());
+    clear_system_time_zone_cache();
+
+    return {};
+}
+
 // https://github.com/unicode-org/icu/blob/main/icu4c/source/tools/tzcode/icuzones
 static constexpr bool is_legacy_non_iana_time_zone(StringView time_zone)
 {

Libraries/LibUnicode/TimeZone.h

@@ -1,11 +1,12 @@
 /*
- * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2024-2025, Tim Flynn <trflynn89@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/Error.h>
 #include <AK/Optional.h>
 #include <AK/String.h>
 #include <AK/Time.h>
@@ -24,6 +25,7 @@ struct TimeZoneOffset {
 };
 
 String current_time_zone();
+ErrorOr<void> set_current_time_zone(StringView);
 void clear_system_time_zone_cache();
 Vector<String> const& available_time_zones();
 Vector<String> available_time_zones_in_region(StringView region);

Tests/LibJS/test-js.cpp

@@ -6,14 +6,11 @@
  */
 
 #include <AK/Enumerate.h>
-#include <LibCore/Environment.h>
 #include <LibJS/Runtime/ArrayBuffer.h>
 #include <LibJS/Runtime/Date.h>
 #include <LibJS/Runtime/TypedArray.h>
 #include <LibTest/JavaScriptTestRunner.h>
 #include <LibUnicode/TimeZone.h>
-#include <stdlib.h>
-#include <time.h>
 
 TEST_ROOT("Libraries/LibJS/Tests");
 
@@ -111,23 +108,13 @@ TESTJS_GLOBAL_FUNCTION(detach_array_buffer, detachArrayBuffer)
 
 TESTJS_GLOBAL_FUNCTION(set_time_zone, setTimeZone)
 {
-    auto current_time_zone = JS::js_null();
+    auto current_time_zone = JS::PrimitiveString::create(vm, Unicode::current_time_zone());
+    auto time_zone = TRY(vm.argument(0).to_string(vm));
 
-    if (auto time_zone = Core::Environment::get("TZ"sv); time_zone.has_value())
-        current_time_zone = JS::PrimitiveString::create(vm, *time_zone);
-
-    if (auto time_zone = vm.argument(0); time_zone.is_null()) {
-        if (auto result = Core::Environment::unset("TZ"sv); result.is_error())
-            return vm.throw_completion<JS::InternalError>(MUST(String::formatted("Could not unset time zone: {}", result.error())));
-    } else {
-        if (auto result = Core::Environment::set("TZ"sv, TRY(time_zone.to_string(vm)), Core::Environment::Overwrite::Yes); result.is_error())
+    if (auto result = Unicode::set_current_time_zone(time_zone); result.is_error())
         return vm.throw_completion<JS::InternalError>(MUST(String::formatted("Could not set time zone: {}", result.error())));
-    }
 
     JS::clear_system_time_zone_cache();
-    Unicode::clear_system_time_zone_cache();
-    tzset();
-
     return current_time_zone;
 }
 

Tests/LibUnicode/TestTimeZone.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2024-2025, Tim Flynn <trflynn89@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -7,29 +7,25 @@
 #include <LibTest/TestCase.h>
 
 #include <AK/StringView.h>
-#include <LibCore/Environment.h>
 #include <LibUnicode/TimeZone.h>
 
 class TimeZoneGuard {
 public:
     explicit TimeZoneGuard(StringView time_zone)
-        : m_time_zone(Core::Environment::get("TZ"sv))
+        : m_time_zone(Unicode::current_time_zone())
     {
-        MUST(Core::Environment::set("TZ"sv, time_zone, Core::Environment::Overwrite::Yes));
-        Unicode::clear_system_time_zone_cache();
+        (void)Unicode::set_current_time_zone(time_zone);
     }
 
     ~TimeZoneGuard()
     {
-        if (m_time_zone.has_value())
-            MUST(Core::Environment::set("TZ"sv, *m_time_zone, Core::Environment::Overwrite::Yes));
-        else
-            MUST(Core::Environment::unset("TZ"sv));
-        Unicode::clear_system_time_zone_cache();
+        MUST(Unicode::set_current_time_zone(m_time_zone));
     }
 
+    String const& time_zone() const { return m_time_zone; }
+
 private:
-    Optional<StringView> m_time_zone;
+    String m_time_zone;
 };
 
 TEST_CASE(current_time_zone)
@@ -38,9 +34,13 @@ TEST_CASE(current_time_zone)
         TimeZoneGuard guard { "America/New_York"sv };
         EXPECT_EQ(Unicode::current_time_zone(), "America/New_York"sv);
     }
+    {
+        TimeZoneGuard guard { "America/Los_Angeles"sv };
+        EXPECT_EQ(Unicode::current_time_zone(), "America/Los_Angeles"sv);
+    }
     {
         TimeZoneGuard guard { "ladybird"sv };
-        EXPECT_EQ(Unicode::current_time_zone(), "UTC"sv);
+        EXPECT_EQ(Unicode::current_time_zone(), guard.time_zone());
     }
 }