diff --git a/app/Console/Commands/CrystalliseEventTimezone.php b/app/Console/Commands/CrystalliseEventTimezone.php
new file mode 100644
index 000000000..02082a936
--- /dev/null
+++ b/app/Console/Commands/CrystalliseEventTimezone.php
@@ -0,0 +1,51 @@
+<?php
+
+namespace App\Console\Commands;
+
+use App\Party;
+use App\Services\DiscourseService;
+use Illuminate\Console\Command;
+use Illuminate\Support\Facades\Log;
+
+class CrystalliseEventTimezone extends Command
+{
+    /**
+     * The name and signature of the console command.
+     *
+     * @var string
+     */
+    protected $signature = 'event:timezones';
+
+    /**
+     * The console command description.
+     *
+     * @var string
+     */
+    protected $description = 'Set timezone on past events';
+
+    /**
+     * Create a new command instance.
+     *
+     * @return void
+     */
+    public function __construct(DiscourseService $discourseService)
+    {
+        parent::__construct();
+        $this->discourseService = $discourseService;
+    }
+
+    /**
+     * Execute the console command.
+     *
+     * @return mixed
+     */
+    public function handle()
+    {
+        $events = Party::past()->where('timezone', null)->get();
+
+        foreach ($events as $event) {
+            $event->timezone = $event->theGroup->timezone;
+            $event->save();
+        }
+    }
+}
diff --git a/app/Console/Kernel.php b/app/Console/Kernel.php
index 84a4a6006..473a39c6e 100644
--- a/app/Console/Kernel.php
+++ b/app/Console/Kernel.php
@@ -43,6 +43,8 @@ protected function schedule(Schedule $schedule)
             ->emailOutputTo(env('SEND_COMMAND_LOGS_TO'), 'tech@therestartproject.org');
 
         $schedule->command('groups:country')->hourly();
+
+        $schedule->command('event:timezones')->hourly();
     }
 
     /**
diff --git a/tests/Unit/Events/TimezoneTest.php b/tests/Unit/Events/TimezoneTest.php
index dc6660cdb..ab3139ebc 100644
--- a/tests/Unit/Events/TimezoneTest.php
+++ b/tests/Unit/Events/TimezoneTest.php
@@ -11,6 +11,9 @@
 use Illuminate\Validation\ValidationException;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 use Tests\TestCase;
+use function PHPUnit\Framework\assertEquals;
+use function PHPUnit\Framework\assertNull;
+use Illuminate\Support\Facades\Artisan;
 
 class TimezoneTest extends TestCase
 {
@@ -221,4 +224,25 @@ public function testTimezoneChangeUpdatesFutureEvents() {
         $party->refresh();
         self::assertEquals('Europe/London', $party->timezone);
     }
+
+    public function testCrystallise() {
+        // Create a past event and check that the scheduled command crystallises the timezone.
+        $g = Group::factory()->create([
+            'timezone' => 'Asia/Samarkand'
+        ]);
+
+        $e = Party::factory()->create([
+            'group' => $g->idgroups,
+            'event_start_utc' => '2021-02-01T10:15:05+05:00',
+            'event_end_utc' => '2021-02-01T13:45:05+05:00',
+            'timezone' => NULL
+        ]);
+
+        // We want to skip the accessor for this test.
+        assertNull($e->getAttributes()['timezone']);
+        Artisan::call('event:timezones');
+
+        $e->refresh();
+        assertEquals('Asia/Samarkand', $e->getAttributes()['timezone']);
+    }
 }