How can I give you format a timestamp by the method find?

I need to filter a timestamp as a code, but only when i filter the query

i work with postgres

<?php

$data = Incidence::find(array(
    "columns" => array("to_char(create_at,'YYYYMMDD')"),
    "order" => "id"
));

how i can to do it?

edited Jul '15

Hi Guy, type of column is string

columns =>"column1, to_char(create_at,'YYYYMMDD')"