diff --git a/app/Http/Controllers/VolunteerController.php b/app/Http/Controllers/VolunteerController.php
index 9b49b49..23083b4 100644
--- a/app/Http/Controllers/VolunteerController.php
+++ b/app/Http/Controllers/VolunteerController.php
@@ -93,8 +93,7 @@ public function checkOut(Request $request, TimeEntry $timeEntry): JsonResponse|R
 	 */
 	public function storeTimeEntry(TimeEntryStoreRequest $request, Event $event, User $user): JsonResponse {
 		// Don't allow an ongoing entry to be created if there already is one
-		$input = $request->safe();
-		if (!isset($input['stop'])) {
+		if (!$request->filled('stop')) {
 			if ($user->timeEntries()->ongoing()->forEvent($event)->exists()) {
 				return response()->json(['error' => 'User already has an ongoing time entry.'], 409);
 			}
diff --git a/app/Http/Requests/TimeEntryStoreRequest.php b/app/Http/Requests/TimeEntryStoreRequest.php
index 83b759d..ad055c4 100644
--- a/app/Http/Requests/TimeEntryStoreRequest.php
+++ b/app/Http/Requests/TimeEntryStoreRequest.php
@@ -43,8 +43,13 @@ public function rules(): array {
 	 */
 	public function passedValidation(): void {
 		$this->merge([
-			'start' => $this->date('start')?->timezone(config('app.timezone')) ?? now(),
-			'stop' => $this->date('stop')?->timezone(config('app.timezone')),
+			'start' => ($this->date('start')?->timezone(config('app.timezone')) ?? now())->toISOString(true),
+			'stop' => $this->date('stop')?->timezone(config('app.timezone'))?->toISOString(true),
 		]);
+
+		$this->validator->setData(array_merge($this->validator->getData(), [
+			'start' => $this->input('start'),
+			'stop' => $this->input('stop'),
+		]));
 	}
 }
diff --git a/tests/Feature/VolunteerTimeEntryStoreTest.php b/tests/Feature/VolunteerTimeEntryStoreTest.php
new file mode 100644
index 0000000..bc52b11
--- /dev/null
+++ b/tests/Feature/VolunteerTimeEntryStoreTest.php
@@ -0,0 +1,59 @@
+<?php
+
+use App\Models\Department;
+use App\Models\Event;
+use App\Models\Role;
+use App\Models\TimeEntry;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+
+uses(RefreshDatabase::class);
+
+it('rejects creating a second ongoing time entry for the same user and event', function () {
+	$manager = User::factory()->create(['role' => Role::Admin]);
+	$volunteer = User::factory()->create();
+	$event = Event::factory()->create();
+	$department = Department::factory()->create(['event_id' => $event->id]);
+
+	$this->actingAs($manager);
+
+	// Mirrors the real clock-in UI, which always sends an explicit 'start' and omits 'stop'.
+	$this->putJson(route('volunteer.time.store', [$event, $volunteer]), [
+		'department_id' => $department->id,
+		'start' => now()->subSecond()->toISOString(),
+	])->assertOk();
+
+	expect(TimeEntry::where('user_id', $volunteer->id)->whereNull('stop')->count())->toBe(1);
+
+	$this->putJson(route('volunteer.time.store', [$event, $volunteer]), [
+		'department_id' => $department->id,
+		'start' => now()->subSecond()->toISOString(),
+	])
+		->assertStatus(409)
+		->assertJson(['error' => 'User already has an ongoing time entry.']);
+
+	expect(TimeEntry::where('user_id', $volunteer->id)->whereNull('stop')->count())->toBe(1);
+});
+
+it('allows creating a time entry with an explicit stop while another is ongoing', function () {
+	$manager = User::factory()->create(['role' => Role::Admin]);
+	$volunteer = User::factory()->create();
+	$event = Event::factory()->create();
+	$department = Department::factory()->create(['event_id' => $event->id]);
+
+	$this->actingAs($manager);
+
+	$this->putJson(route('volunteer.time.store', [$event, $volunteer]), [
+		'department_id' => $department->id,
+		'start' => now()->subSecond()->toISOString(),
+	])->assertOk();
+
+	$this->putJson(route('volunteer.time.store', [$event, $volunteer]), [
+		'department_id' => $department->id,
+		'start' => now()->subHour()->toISOString(),
+		'stop' => now()->toISOString(),
+	])->assertOk();
+
+	expect(TimeEntry::where('user_id', $volunteer->id)->count())->toBe(2);
+	expect(TimeEntry::where('user_id', $volunteer->id)->whereNull('stop')->count())->toBe(1);
+});
\ No newline at end of file