diff --git a/core/modules/datetime/datetime.module b/core/modules/datetime/datetime.module
index 4760b38..245d0c4 100644
--- a/core/modules/datetime/datetime.module
+++ b/core/modules/datetime/datetime.module
@@ -13,6 +13,8 @@
  * @deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.x. Use
  * \Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface::STORAGE_TIMEZONE
  * instead.
+ *
+ * @see https://www.drupal.org/node/2912980
  */
 const DATETIME_STORAGE_TIMEZONE = 'UTC';
 
@@ -22,6 +24,8 @@
  * @deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.x. Use
  * \Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface::DATETIME_STORAGE_FORMAT
  * instead.
+ *
+ * @see https://www.drupal.org/node/2912980
  */
 const DATETIME_DATETIME_STORAGE_FORMAT = 'Y-m-d\TH:i:s';
 
@@ -31,6 +35,8 @@
  * @deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.x. Use
  * \Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface::DATE_STORAGE_FORMAT
  * instead.
+ *
+ * @see https://www.drupal.org/node/2912980
  */
 const DATETIME_DATE_STORAGE_FORMAT = 'Y-m-d';
 
diff --git a/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
index 45d8846..3264069 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
@@ -109,10 +109,10 @@ public static function generateSampleValue(FieldDefinitionInterface $field_defin
     // type.
     $timestamp = REQUEST_TIME - mt_rand(0, 86400 * 365);
     if ($type == DateTimeItem::DATETIME_TYPE_DATE) {
-      $values['value'] = gmdate(DateTimeItemInterface::DATE_STORAGE_FORMAT, $timestamp);
+      $values['value'] = gmdate(static::DATE_STORAGE_FORMAT, $timestamp);
     }
     else {
-      $values['value'] = gmdate(DateTimeItemInterface::DATETIME_STORAGE_FORMAT, $timestamp);
+      $values['value'] = gmdate(static::DATETIME_STORAGE_FORMAT, $timestamp);
     }
     return $values;
   }