diff --git a/Libraries/LibUnicode/TimeZone.cpp b/Libraries/LibUnicode/TimeZone.cpp
index d1bc1fff42a..acd43715c8d 100644
--- a/Libraries/LibUnicode/TimeZone.cpp
+++ b/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)
 {
diff --git a/Libraries/LibUnicode/TimeZone.h b/Libraries/LibUnicode/TimeZone.h
index 518589d1fd7..66f1601ff15 100644
--- a/Libraries/LibUnicode/TimeZone.h
+++ b/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);
diff --git a/Tests/LibJS/test-js.cpp b/Tests/LibJS/test-js.cpp
index 1ada18db2f8..eeeb2b88393 100644
--- a/Tests/LibJS/test-js.cpp
+++ b/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())
-            return vm.throw_completion<JS::InternalError>(MUST(String::formatted("Could not set time zone: {}", result.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;
 }
 
diff --git a/Tests/LibUnicode/TestTimeZone.cpp b/Tests/LibUnicode/TestTimeZone.cpp
index 208a0b90137..c79a200124f 100644
--- a/Tests/LibUnicode/TestTimeZone.cpp
+++ b/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());
     }
 }