Export Contacts: Created Date and Modified Date are not retrieved

Export Contacts > Primary Fields includes columns for modified_date and created_date - but the data is not actually retrieved from the DB. These fields are also available to 'Select Fields for Export' - but also do not produce output. (I verified the corresponding DB values in civicrm_contact.modifed_date and .created_date - and dates were populated for the contacts I included in the export).