To get post publish date outside the loop in wordpress use the following code.

$all_posts = get_posts(array('numberposts'=>-1, 'post_status'=> 'publish')); //Get all published posts
foreach ($all_posts as $single_post){
    echo get_the_time('Y-m-d', $single_post->ID); //The date is on Y-m-d format
    echo '<br />' ;
}

Hope this code will help you.
Thanks