Add a “New Post” Label to Recently Published WordPress Posts

/**
 * Add a "New" label to recently published posts
 */
function custom_add_new_post_label( $title, $post_id ) {

    if ( is_admin() || get_post_type( $post_id ) !== 'post' ) {
        return $title;
    }

    $published_time = get_post_time( 'U', true, $post_id );
    $days_ago       = ( time() - $published_time ) / DAY_IN_SECONDS;

    if ( $days_ago <= 3 ) {
        $title .= ' <span class="new-post-label">New</span>';
    }

    return $title;
}

add_filter( 'the_title', 'custom_add_new_post_label', 10, 2 );