“New” Badge for Recent Posts

add_filter( 'the_title', function ( $title, $id ) {

	if ( ! is_admin() && is_single( $id ) ) {

		$number_of_days = 7;

		$post_date      = get_the_date( 'U', $id );

		$current_date   = current_time( 'timestamp' );

		$date_diff      = $current_date - $post_date;

		if ( $date_diff < $number_of_days * DAY_IN_SECONDS ) {

			$title .= ' <span class="new-badge">New</span>';

		}

	}

	return $title;

}, 10, 2 );

add_action( 'wp_head', function () {

	echo '

        <style>

            .new-badge {

                background-color: #ff0000; 

                color: #ffffff; 

                padding: 2px 5px; 

                font-size: 12px; 

                border-radius: 3px;

                margin-left: 5px;

            }

        </style>

    ';

} );