Provide a TimestampItemNormalizer with an option to return RFC3339 formatted date.

modules/core/csv/farm_csv.services.yml

@@ -13,3 +13,7 @@ services:
     tags:
       - { name: normalizer, priority: 10 }
     arguments: ['@entity.repository']
+  farm_csv.normalizer.timestamp_item:
+    class: Drupal\farm_csv\Normalizer\TimestampItemNormalizer
+    tags:
+      - { name: normalizer, priority: 10 }

modules/core/csv/src/Normalizer/TimestampItemNormalizer.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\farm_csv\Normalizer;
+
+use Drupal\serialization\Normalizer\TimestampItemNormalizer as CoreTimestampItemNormalizer;
+
+/**
+ * Normalizes timestamp fields for farmOS CSV exports.
+ */
+class TimestampItemNormalizer extends CoreTimestampItemNormalizer {
+
+  /**
+   * The supported format.
+   */
+  const FORMAT = 'csv';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = []): array|string|int|float|bool|\ArrayObject|NULL {
+    $data = parent::normalize($object, $format, $context);
+
+    // Return the RFC3339 formatted date, if desired.
+    if (!empty($context['rfc3339_dates'])) {
+      return $data['value'];
+    }
+
+    return $data;
+  }
+
+}