diff --git a/app/Modules/Files/Http/Controllers/Api/FilesController.php b/app/Modules/Files/Http/Controllers/Api/FilesController.php
index 0f4a083f..3e3d7c43 100644
--- a/app/Modules/Files/Http/Controllers/Api/FilesController.php
+++ b/app/Modules/Files/Http/Controllers/Api/FilesController.php
@@ -21,9 +21,12 @@ use App\Modules\Files\Models\Folder;
 use App\Modules\Files\Storage\ResolvingUploadDisk;
 use App\Modules\Files\Uploads\StoreUploadedFile;
 use App\Modules\Files\Uploads\UploadExtensionPolicy;
+use App\Modules\Platform\Localization\LocalDay;
+use App\Modules\Platform\Localization\TimezoneRegistry;
 use App\Modules\Platform\Settings\Setting;
 use App\Modules\Platform\Settings\Settings;
 use App\Support\Rules;
+use Carbon\Carbon;
 use Closure;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Relations\Relation;
@@ -59,6 +62,7 @@ class FilesController extends Controller
         private readonly ActivityLogger $activity,
         private readonly CommentingRules $commenting,
         private readonly StaffLibraryScope $scope,
+        private readonly TimezoneRegistry $timezones,
     ) {}
 
     /**
@@ -263,6 +267,12 @@ class FilesController extends Controller
      * without the matching permission leaves that field untouched rather
      * than failing the whole request, which mirrors the web interface.
      *
+     * `expires_at` accepts either a calendar day (`2026-09-12`) or a full
+     * timestamp. A day means the end of that day in the caller's timezone,
+     * which is what the same value means on the web and what the file's
+     * own `expires_at` reads back as; a timestamp is taken as the instant
+     * it names.
+     *
      * `commentable` only has an effect while the installation's comment
      * setting is "only files marked as commentable"; under any other
      * setting it is ignored, again rather than failing.
@@ -306,7 +316,7 @@ class FilesController extends Controller
         $attributes = array_intersect_key($validated, array_flip(['name', 'description', 'folder_id']));
 
         if (array_key_exists('expires_at', $validated) && $user->can('set_file_expiration_date')) {
-            $attributes['expires_at'] = $validated['expires_at'];
+            $attributes['expires_at'] = $this->expiryInstant($validated['expires_at'], $user);
         }
 
         if (array_key_exists('download_limit', $validated) && $user->can('limit_downloads')) {
@@ -356,4 +366,29 @@ class FilesController extends Controller
 
         return response()->json(status: 204);
     }
+
+    /**
+     * What an `expires_at` value means.
+     *
+     * A bare `YYYY-MM-DD` is a calendar day, and a calendar day ends where
+     * the person naming it lives — the same rule the web form's date input
+     * gets from FilesController::expiryInstant. Stored as it arrives it
+     * would be midnight UTC instead, so a file asked to expire on the 12th
+     * would die at the *start* of the 12th, and for a caller west of
+     * Greenwich partway through the 11th.
+     *
+     * Anything carrying a time is an instant the caller named on purpose
+     * and is stored as it arrives, unchanged from before: the API can
+     * express a moment, and a date input cannot.
+     */
+    private function expiryInstant(?string $value, User $setter): ?Carbon
+    {
+        if ($value === null) {
+            return null;
+        }
+
+        return preg_match('/^\d{4}-\d{2}-\d{2}$/', $value) === 1
+            ? LocalDay::end($value, $this->timezones->resolve($setter))
+            : Carbon::parse($value);
+    }
 }
diff --git a/docs/api/openapi.json b/docs/api/openapi.json
index aa87ab86..f35c37d1 100644
--- a/docs/api/openapi.json
+++ b/docs/api/openapi.json
@@ -2076,7 +2076,7 @@
             },
             "patch": {
                 "operationId": "files.update",
-                "description": "Only the fields present in the request are changed; omitting one\nleaves it as it was.\n\nSome fields need a permission of their own \u2014 `expires_at` needs\n`set_file_expiration_date`, `public` needs `upload_public`, and\n`categories` needs `set_file_categories`. Sending one of those\nwithout the matching permission leaves that field untouched rather\nthan failing the whole request, which mirrors the web interface.\n\n`commentable` only has an effect while the installation's comment\nsetting is \"only files marked as commentable\"; under any other\nsetting it is ignored, again rather than failing.\n\nRequires a token with any of these abilities: `edit_files`, `edit_others_files`.",
+                "description": "Only the fields present in the request are changed; omitting one\nleaves it as it was.\n\nSome fields need a permission of their own \u2014 `expires_at` needs\n`set_file_expiration_date`, `public` needs `upload_public`, and\n`categories` needs `set_file_categories`. Sending one of those\nwithout the matching permission leaves that field untouched rather\nthan failing the whole request, which mirrors the web interface.\n\n`expires_at` accepts either a calendar day (`2026-09-12`) or a full\ntimestamp. A day means the end of that day in the caller's timezone,\nwhich is what the same value means on the web and what the file's\nown `expires_at` reads back as; a timestamp is taken as the instant\nit names.\n\n`commentable` only has an effect while the installation's comment\nsetting is \"only files marked as commentable\"; under any other\nsetting it is ignored, again rather than failing.\n\nRequires a token with any of these abilities: `edit_files`, `edit_others_files`.",
                 "summary": "Update a file's metadata",
                 "tags": [
                     "Files"
diff --git a/tests/Feature/Api/FilesWriteTest.php b/tests/Feature/Api/FilesWriteTest.php
index a9663053..b2d5088a 100644
--- a/tests/Feature/Api/FilesWriteTest.php
+++ b/tests/Feature/Api/FilesWriteTest.php
@@ -124,6 +124,47 @@ test('a file can be updated field by field', function () {
         ->and($file->description)->toBe($file->getOriginal('description'));
 });
 
+test('a calendar day means the end of that day where the caller lives', function () {
+    // The same value on the web means the end of the 12th (LocalDay::end via
+    // FilesController::expiryInstant). Stored as it arrives it is midnight
+    // UTC, so the file would die at the *start* of the 12th instead.
+    $this->admin->update(['timezone' => 'Europe/Berlin']);
+    $file = File::factory()->create(['uploaded_by' => $this->admin->id]);
+
+    $this->withToken($this->token)->patchJson("/api/v1/files/{$file->id}", [
+        'expires_at' => '2026-09-12',
+    ])->assertOk();
+
+    // 23:59:59 on the 12th in Berlin is 21:59:59Z.
+    expect($file->refresh()->expires_at?->toIso8601String())->toBe('2026-09-12T21:59:59+00:00');
+});
+
+test('a timestamp is stored as the instant it names', function () {
+    // The half that must not change: an API caller can name a moment, and
+    // naming one is not the same as naming a day.
+    $this->admin->update(['timezone' => 'Europe/Berlin']);
+    $file = File::factory()->create(['uploaded_by' => $this->admin->id]);
+
+    $this->withToken($this->token)->patchJson("/api/v1/files/{$file->id}", [
+        'expires_at' => '2026-09-12T08:30:00+00:00',
+    ])->assertOk();
+
+    expect($file->refresh()->expires_at?->toIso8601String())->toBe('2026-09-12T08:30:00+00:00');
+});
+
+test('clearing the expiry still clears it', function () {
+    $file = File::factory()->create([
+        'uploaded_by' => $this->admin->id,
+        'expires_at' => now()->addWeek(),
+    ]);
+
+    $this->withToken($this->token)->patchJson("/api/v1/files/{$file->id}", [
+        'expires_at' => null,
+    ])->assertOk();
+
+    expect($file->refresh()->expires_at)->toBeNull();
+});
+
 test('fields the caller lacks permission for are left alone rather than refused', function () {
     // Mirrors the web controller: a user who may edit a file but not set
     // expiry dates still gets to rename it.