You need to format date like "Y-m-d H:i:s" in order to work with MySQL datetime field.
i.e. :
$lastupdated = date('Y-m-d H:i:s');
From documentation :
The DATETIME type is used for values that contain both date and time parts. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'.
