LibCore: Keep ICU and libc time zones in sync

Add Core::TimeZone as the single entry point for changing the
current time zone. This updates both ICU and the process TZ/tzset
state so libc-backed helpers such as AK::UnixDateTime::to_string()
agree with JS/ICU time zone state. Previously only the ICU timezone
would be updated, which could result in inconsistent results being
returned.

Libraries/LibCore/CMakeLists.txt

@@ -25,6 +25,7 @@ set(SOURCES
     SystemServerTakeover.cpp
     ThreadEventQueue.cpp
     Timer.cpp
+    TimeZone.cpp
     Version.cpp
 )
 

Libraries/LibCore/TimeZone.cpp

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2026, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/Environment.h>
+#include <LibCore/TimeZone.h>
+#include <LibUnicode/TimeZone.h>
+#include <time.h>
+
+namespace Core::TimeZone {
+
+ErrorOr<void> set_current_time_zone(StringView time_zone)
+{
+    TRY(Unicode::set_current_time_zone(time_zone));
+    TRY(Core::Environment::set("TZ"sv, time_zone, Core::Environment::Overwrite::Yes));
+    tzset();
+    return {};
+}
+
+String current_time_zone()
+{
+    return Unicode::current_time_zone();
+}
+
+}

Libraries/LibCore/TimeZone.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2026, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <LibCore/Export.h>
+
+namespace Core::TimeZone {
+
+CORE_API ErrorOr<void> set_current_time_zone(StringView);
+
+CORE_API String current_time_zone();
+
+}

Libraries/LibWeb/Internals/Internals.cpp

@@ -6,6 +6,7 @@
  */
 
 #include <AK/JsonObject.h>
+#include <LibCore/TimeZone.h>
 #include <LibGfx/Cursor.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/Date.h>
@@ -13,7 +14,6 @@
 #include <LibJS/Runtime/Reference.h>
 #include <LibJS/Runtime/VM.h>
 #include <LibURL/Parser.h>
-#include <LibUnicode/TimeZone.h>
 #include <LibWeb/ARIA/AriaData.h>
 #include <LibWeb/ARIA/StateAndProperties.h>
 #include <LibWeb/Bindings/Internals.h>
@@ -207,9 +207,9 @@ WebIDL::ExceptionOr<void> Internals::mark_as_garbage(StringView variable_name)
 
 WebIDL::ExceptionOr<String> Internals::set_time_zone(StringView time_zone)
 {
-    auto current_time_zone = Unicode::current_time_zone();
+    auto current_time_zone = Core::TimeZone::current_time_zone();
 
-    if (auto result = Unicode::set_current_time_zone(time_zone); result.is_error())
+    if (auto result = Core::TimeZone::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();

Services/WebContent/main.cpp

@@ -11,6 +11,7 @@
 #include <LibCore/Process.h>
 #include <LibCore/Resource.h>
 #include <LibCore/System.h>
+#include <LibCore/TimeZone.h>
 #include <LibCrypto/OpenSSLForward.h>
 #include <LibGfx/Font/FontDatabase.h>
 #include <LibGfx/Font/PathFontProvider.h>
@@ -183,7 +184,7 @@ ErrorOr<int> ladybird_main(Main::Arguments arguments)
     }
 
     if (!default_time_zone.is_empty()) {
-        if (auto result = Unicode::set_current_time_zone(default_time_zone); result.is_error())
+        if (auto result = Core::TimeZone::set_current_time_zone(default_time_zone); result.is_error())
             dbgln("Failed to set default time zone: {}", result.error());
     }
 

Tests/LibJS/test-js.cpp

@@ -7,13 +7,13 @@
 
 #include <AK/Enumerate.h>
 #include <AK/StringView.h>
+#include <LibCore/TimeZone.h>
 #include <LibJS/Runtime/ArrayBuffer.h>
 #include <LibJS/Runtime/Date.h>
 #include <LibJS/Runtime/FinalizationRegistry.h>
 #include <LibJS/Runtime/TypedArray.h>
 #include <LibJS/Runtime/ValueInlines.h>
 #include <LibTest/JavaScriptTestRunner.h>
-#include <LibUnicode/TimeZone.h>
 
 TEST_ROOT("Tests/LibJS/Runtime");
 
@@ -142,10 +142,10 @@ TESTJS_GLOBAL_FUNCTION(detach_array_buffer, detachArrayBuffer)
 
 TESTJS_GLOBAL_FUNCTION(set_time_zone, setTimeZone)
 {
-    auto current_time_zone = JS::PrimitiveString::create(vm, Unicode::current_time_zone());
+    auto current_time_zone = JS::PrimitiveString::create(vm, Core::TimeZone::current_time_zone());
     auto time_zone = TRY(vm.argument(0).to_string(vm));
 
-    if (auto result = Unicode::set_current_time_zone(time_zone); result.is_error())
+    if (auto result = Core::TimeZone::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();

Tests/LibWeb/Text/expected/wpt-import/html/dom/documents/resource-metadata-management/document-lastModified.txt

@@ -0,0 +1,6 @@
+Harness status: OK
+
+Found 1 tests
+
+1 Pass
+Pass	lastModified should return the last modified date and time

Tests/LibWeb/Text/input/wpt-import/html/dom/documents/resource-metadata-management/document-lastModified.html

@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>document.lastModified</title>
+<link rel="author" title="Denis Ah-Kang" href="mailto:denis@w3.org">
+<link rel=help href="https://html.spec.whatwg.org/multipage/#resource-metadata-management">
+<script src="../../../../resources/testharness.js"></script>
+<script src="../../../../resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+  test(function(){
+    var date = new Date("Thu, 01 Jan 1970 01:23:45 GMT");
+    var result = ('0' + (date.getMonth()+1)).slice(-2) + '/' + ('0' + date.getDate()).slice(-2) + '/' + date.getFullYear() + " " + [date.getHours(),date.getMinutes(),date.getSeconds()].map(function(n){return ("0" + n).slice(-2);}).join(":");
+    assert_equals(document.lastModified, result);
+  }, "lastModified should return the last modified date and time");
+</script>

Tests/LibWeb/Text/input/wpt-import/html/dom/documents/resource-metadata-management/document-lastModified.html.headers

@@ -0,0 +1 @@
+Last-Modified: Thu, 01 Jan 1970 01:23:45 GMT