fix date column in export (#5308)

src/Export/Package/CellFormatter/DateStringFormatter.php

@@ -0,0 +1,26 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Export\Package\CellFormatter;
+
+final class DateStringFormatter implements CellFormatterInterface
+{
+    public function formatValue(mixed $value): mixed
+    {
+        if ($value instanceof \DateTimeInterface) {
+            return $value->format('Y-m-d');
+        }
+
+        if ($value === null) {
+            return null;
+        }
+
+        throw new \InvalidArgumentException('Only DateTimeInterface can be formatted');
+    }
+}