Add Offset to Wordpress RSS Feed

functions.php

<?php
    function rss_pre_get_posts($query) {
        if (isset($_GET['days_past']) && is_feed()) {
            $query->set('date_query', array(array('after' => $_GET['days_past'] . ' days ago',)));
        }
        if (isset($_GET['offset']) && is_feed()) {
            $query->set('offset', $_GET['offset']);
        }
        if (isset($_GET['cat']) && is_feed()) {
            $query->set('cat', $_GET['cat']);
        }

        return $query;
    }

    add_action('pre_get_posts', 'rss_pre_get_posts');