Accept UTC starts in timezone-aware RRULE schedules

src/Drivers/RruleSchedule.php

@@ -53,7 +53,9 @@ protected function toRfcString(): string
         [$expression, $dtstart] = RruleExpression::extractDtstart($this->toExpression());
 
         if ($dtstart) {
-            $tzid = $this->timezone ? ";TZID={$this->timezone}" : '';
+            $tzid = $this->timezone && ! str_ends_with($dtstart, 'Z')
+                ? ";TZID={$this->timezone}"
+                : '';
 
             return "DTSTART{$tzid}:{$dtstart}\nRRULE:{$expression}";
         }

tests/Drivers/RruleScheduleTest.php

@@ -56,6 +56,26 @@
     expect($next->timezone('UTC')->format('Y-m-d H:i:s'))->toBe('2026-05-02 13:00:00');
 });
 
+it('preserves a UTC start when a timezone is configured', function ($timezone, $limit) {
+    $expression = "FREQ=DAILY;{$limit};DTSTART=20260501T090000Z";
+    $driver = new RruleSchedule($expression, $timezone);
+
+    $next = $driver->getNextOccurrence(Carbon::parse('2026-05-02 08:00:00', 'UTC'));
+
+    expect($next?->utc()->toDateTimeString())->toBe('2026-05-02 09:00:00')
+        ->and($driver->toExpression())->toBe($expression)
+        ->and($driver->getNextOccurrence(Carbon::parse('2026-05-03 09:00:00', 'UTC')))->toBeNull();
+})->with(['America/New_York', 'Australia/Sydney'])->with(['COUNT=3', 'UNTIL=20260503T090000Z']);
+
+it('preserves a UTC start when the timezone is set via method', function () {
+    $driver = (new RruleSchedule('FREQ=DAILY;DTSTART=20260501T090000Z'))
+        ->setTimezone('America/New_York');
+
+    $next = $driver->getNextOccurrence(Carbon::parse('2026-05-02 08:00:00', 'UTC'));
+
+    expect($next?->utc()->toDateTimeString())->toBe('2026-05-02 09:00:00');
+});
+
 it('serializes to expression', function () {
     Carbon::setTestNow('2026-05-02 10:00:00');
 

tests/ScheduleTest.php

@@ -187,6 +187,27 @@
         ->and($schedule->expression)->toBe('FREQ=DAILY;COUNT=2;DTSTART=20260501T090000');
 })->with([RruleSchedule::class, RecurrSchedule::class]);
 
+it('preserves an explicit UTC recurrence start across reloads and timezone changes', function () {
+    $expression = 'FREQ=DAILY;DTSTART=20260501T090000Z';
+    $schedule = SchedulableModel::create()->addSchedule(
+        new RruleSchedule($expression, 'America/New_York')
+    );
+
+    expect($schedule->refresh()->next_run_at->toDateTimeString())->toBe('2026-05-03 09:00:00')
+        ->and($schedule->expression)->toBe($expression);
+
+    $schedule->update(['timezone' => 'Australia/Sydney']);
+
+    expect($schedule->refresh()->next_run_at->toDateTimeString())->toBe('2026-05-03 09:00:00')
+        ->and($schedule->expression)->toBe($expression);
+
+    Carbon::setTestNow('2026-05-03 09:01:00');
+
+    $schedule->advance(now());
+
+    expect($schedule->refresh()->next_run_at->toDateTimeString())->toBe('2026-05-04 09:00:00');
+});
+
 it('resolves the driver from a stored schedule', function () {
     $schedule = SchedulableModel::create()->addSchedule(new CronSchedule('0 12 * * *'));