All Posts admin screen – show time underneath the ‘Date’?

Add this code to functions.php of your active theme:

function wpse_posts_list_date_format( $time, $post ) {
    return $post->post_date;
}
add_filter( 'post_date_column_time', 'wpse_posts_list_date_format', 10, 2 );

The callback function receives 4 parameters, but we need only 2.