fix date column in export (#5308)

tests/Export/Package/CellFormatter/DateStringFormatterTest.php

@@ -0,0 +1,44 @@
+<?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\Tests\Export\Package\CellFormatter;
+
+use App\Export\Package\CellFormatter\DateStringFormatter;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @covers \App\Export\Package\CellFormatter\DateStringFormatter
+ */
+class DateStringFormatterTest extends TestCase
+{
+    public function testFormatValueReturnsFormattedDateForDateTime(): void
+    {
+        $formatter = new DateStringFormatter();
+        $date = new \DateTime('2023-10-01 12:37');
+        $result = $formatter->formatValue($date);
+        $this->assertIsString($result);
+        $this->assertEquals('2023-10-01', $result);
+    }
+
+    public function testFormatValueReturnsNullForNullValue(): void
+    {
+        $formatter = new DateStringFormatter();
+        $result = $formatter->formatValue(null);
+        self::assertNull($result);
+    }
+
+    public function testFormatValueThrowsExceptionForNonDateTime(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('Only DateTimeInterface can be formatted');
+
+        $formatter = new DateStringFormatter();
+        $formatter->formatValue('not a date');
+    }
+}