Wordpress - How to get the post publish date outside the loop?

get_the_date must be used inside the Loop. For outside the loop use get_the_time.

$posts = get_posts(array('numberposts'=>-1)); //Get all published posts
foreach ($posts as $post){
    echo get_the_time('Y-m-d', $post->ID); //Echos date in Y-m-d format.
}

Consider replacing 'Y-m-d' in this example with get_option('date_format') as this will display the date as per your date format setting in wp-admin.

You can use get_post() or get_post_field() for this, both work outside the loop.

$post_object = get_post($id);
$post_date = date( 'F jS, Y', strtotime( $post_object->post_date ) );

A full list of values returned by get_post:

WP_Post Object
(
    [ID] =>
    [post_author] =>
    [post_date] => 
    [post_date_gmt] => 
    [post_content] => 
    [post_title] => 
    [post_excerpt] => 
    [post_status] =>
    [comment_status] =>
    [ping_status] => 
    [post_password] => 
    [post_name] =>
    [to_ping] => 
    [pinged] => 
    [post_modified] => 
    [post_modified_gmt] =>
    [post_content_filtered] => 
    [post_parent] => 
    [guid] => 
    [menu_order] =>
    [post_type] =>
    [post_mime_type] => 
    [comment_count] =>
    [filter] =>
)