diff --git a/.github/actions/setup-php/action.yml b/.github/actions/setup-php/action.yml
index cf4df451c2..0b35b8c7d6 100644
--- a/.github/actions/setup-php/action.yml
+++ b/.github/actions/setup-php/action.yml
@@ -15,7 +15,7 @@ runs:
       with:
         php-version: 8.5
         tools: composer:v2
-        extensions: pdo_pgsql
+        extensions: intl, pdo_pgsql
         coverage: ${{ inputs.coverage }}
         ini-values: zend.assertions=1, memory_limit=-1
 
diff --git a/README.md b/README.md
index 7d1254b35f..7a0e8e8922 100644
--- a/README.md
+++ b/README.md
@@ -20,7 +20,7 @@ This is the source code of **[dev-null.sh](https://dev-null.sh)**, a social netw
 
 ## Contributing
 
-Requires PHP 8.5+, Composer, Bun, and PostgreSQL 17+ with `pgvector` and `pg_trgm`. Create a `devnull` database, then:
+Requires PHP 8.5+ with `intl`, Composer, Bun, and PostgreSQL 17+ with `pgvector` and `pg_trgm`. Create a `devnull` database, then:
 
 ```bash
 git clone https://github.com/dev-null-sh/dev-null.sh.git
diff --git a/tests/Feature/EventThemeTest.php b/tests/Feature/EventThemeTest.php
index c999a1d840..d852d86337 100644
--- a/tests/Feature/EventThemeTest.php
+++ b/tests/Feature/EventThemeTest.php
@@ -62,6 +62,22 @@
         ->assertSee('data-event-theme="'.$choice.'"', false);
 })->with(['halloween', 'christmas', 'newyear']);
 
+it('reads the local event date from browser time zone aliases', function (string $timeZone, string $date): void {
+    $this->travelTo(Date::parse($date));
+
+    $this->withUnencryptedCookie('time_zone', $timeZone)
+        ->get(route('home'))
+        ->assertOk()
+        ->assertSee('data-auto-event-theme="halloween"', false)
+        ->assertSee('data-event-theme="halloween"', false)
+        ->assertInertia(fn ($page) => $page
+            ->where('eventTheme', 'halloween')
+            ->where('clock.timeZone', $timeZone));
+})->with([
+    'a former name browsers still report' => ['Asia/Calcutta', '2026-10-28 20:00:00 UTC'],
+    'a legacy regional alias' => ['US/Eastern', '2026-11-02 02:00:00 UTC'],
+]);
+
 it('keeps the usual colors for readers who picked default', function (): void {
     Date::setTestNow('2026-12-25 12:00:00');