Use SS_Datetime instead of Datetime. Allows the usage of Object::useCustomClass('SS_Datetime', 'LocalDatetime');

code/BlogEntry.php

@@ -6,7 +6,7 @@
  */
 class BlogEntry extends Page {
 	static $db = array(
-		"Date" => "Datetime",
+		"Date" => "SS_Datetime",
 		"Author" => "Text",
 		"Tags" => "Text"
 	);