Display posts of the last 7 days

I’m trying to display the 5 best rated posts of the last week (7 days) on my website, however I can’t seem to figure out how to display them.

Here’s what I’ve achieved so far but it doesn’t seem to work:

<?php $slider_query = new WP_Query('posts_per_page=5&cat=3&orderby=highest_rated&order=desc'); ?>

<?php

$mylimit = 7 * 86400; //days * seconds per day

while ($slider_query->have_posts()) : $slider_query->the_post();

    $post_age = date('U') - get_post_time('U');

    if ($post_age < $mylimit) { 
?>

//The Post

<?php } ?>

<?php endwhile;?>

In addition to birgire’s solution, as of WordPress 3.7, you can use Date parameters.

Your arguments would look like this to filter posts from the last 7 days:

$args = array(
    'post_type' => 'post',
    'post_status' => 'publish',
    'orderby' => 'date',
    'order' => 'DESC',

    // Using the date_query to filter posts from last week
    'date_query' => array(
        array(
            'after' => '1 week ago'
        )
    )
);