Adds a new class ("new-post") if a post is fresh (within X days). For WordPress.

gistfile1.php

<?php
/**
 * @author Maor Chasen <maor@maorchasen.com>
 */


function mc_497346850334383( $classes, $class, $post_id ) {
	$post = get_post( $post_id );

	if ( ! $post )
		return $classes;

	$days_fresh = 5; // Posts that are older than (or equal to) N days won't be considered new

	$post_date 	  = new DateTime( mysql2date( 'Y-m-d', $post->post_date ) );
	$current_date = new DateTime( date( 'Y-m-d' ) );

	$diff = absint( $current_date->diff( $post_date )->format( '%a' ) );

	if ( $diff <= $days_fresh )
		$classes[] = 'new-post';

	return $classes;
}
add_filter( 'post_class', 'mc_497346850334383', 10, 3 );