diff --git a/src/Form/Extension/DateTimeModelTimezoneExtension.php b/src/Form/Extension/DateTimeModelTimezoneExtension.php
new file mode 100644
index 00000000..3c4818ea
--- /dev/null
+++ b/src/Form/Extension/DateTimeModelTimezoneExtension.php
@@ -0,0 +1,79 @@
+<?php
+/*
+ * This file is part of Part-DB (https://github.com/Part-DB/Part-DB-symfony).
+ *
+ *  Copyright (C) 2019 - 2025 Jan Böhmer (https://github.com/jbtronics)
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Affero General Public License as published
+ *  by the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Affero General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Affero General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+namespace App\Form\Extension;
+
+use Symfony\Component\Form\AbstractTypeExtension;
+use Symfony\Component\Form\Extension\Core\Type\DateTimeType;
+use Symfony\Component\Form\Extension\Core\Type\DateType;
+use Symfony\Component\Form\FormBuilderInterface;
+use Symfony\Component\Form\FormEvent;
+use Symfony\Component\Form\FormEvents;
+
+/**
+ * Catches timezone mismatches between a DateTimeInterface model value and the effective
+ * model_timezone configured on the field.
+ *
+ * Doctrine's UTCDateTimeImmutableType always returns UTC DateTimeImmutable objects, so any
+ * date/datetime field that omits `model_timezone: 'UTC'` will silently corrupt stored values
+ * (the transformer treats the UTC instant as if it were in the user's local timezone).
+ * This extension throws a \LogicException early so the mistake is caught at development time.
+ */
+class DateTimeModelTimezoneExtension extends AbstractTypeExtension
+{
+    public static function getExtendedTypes(): iterable
+    {
+        return [DateTimeType::class, DateType::class];
+    }
+
+    public function buildForm(FormBuilderInterface $builder, array $options): void
+    {
+        $builder->addEventListener(FormEvents::POST_SET_DATA, static function (FormEvent $event) use ($options): void {
+            $data = $event->getData();
+
+            if (!$data instanceof \DateTimeInterface) {
+                return;
+            }
+
+            // Resolve the effective model timezone: explicit option or the PHP default set at build time.
+            // This mirrors what BaseDateTimeTransformer does in its constructor.
+            $modelTimezone = $options['model_timezone'] ?? date_default_timezone_get();
+
+            $dataOffset  = $data->getTimezone()->getOffset($data);
+            $modelOffset = (new \DateTimeZone($modelTimezone))->getOffset($data);
+
+            if ($dataOffset !== $modelOffset) {
+                throw new \LogicException(sprintf(
+                    'Form field "%s" received a %s with timezone "%s" (UTC offset %+d s), '
+                    . 'but the effective model_timezone is "%s" (UTC offset %+d s). '
+                    . 'Set the "model_timezone" option to match the timezone of your data source.',
+                    $event->getForm()->getName(),
+                    get_debug_type($data),
+                    $data->getTimezone()->getName(),
+                    $dataOffset,
+                    $modelTimezone,
+                    $modelOffset
+                ));
+            }
+        });
+    }
+}
diff --git a/src/Form/Part/PartLotType.php b/src/Form/Part/PartLotType.php
index fc330bb1..ef49c57e 100644
--- a/src/Form/Part/PartLotType.php
+++ b/src/Form/Part/PartLotType.php
@@ -115,8 +115,10 @@ class PartLotType extends AbstractType
         $builder->add('last_stocktake_at', DateTimeType::class, [
             'label' => 'part_lot.edit.last_stocktake_at',
             'widget' => 'single_text',
+            'model_timezone' => 'UTC', // The database stores the datetime in UTC, so we need to set the model timezone to UTC
             'disabled' => !$this->security->isGranted('@parts_stock.stocktake'),
             'required' => false,
+            'with_seconds' => true,
         ]);
     }