过滤内容post_class():

add_filter( \'post_class\', function( $classes ) {

    if ( is_singular() )
        return $classes;

    // now minus last mod time in seconds
    $diff = time() - mysql2date( \'U\', $post->post_date );

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

    return $classes;
});

.new-post {
    padding-left: 20px;
    background-image: url(new.png) left top no-repeat;
}