If you don’t add a post_date then WordPress fills it automatically with the current date and time.
To set another date and time [ Y-m-d H:i:s ]
is the right structure. An example below with your code.
$postdate="2010-02-23 18:57:33";
$new_post = array(
'post_title' => $title,
'post_content' => $description,
'post_date' => $postdate,
'post_status' => 'publish',
'post_parent' => $parent_id,
'post_author' => get_current_user_id(),
);
//SAVE THE POST
$pid = wp_insert_post($new_post);