functions.php

<?php
/** 
 * Display Posts Shortcode - Offset Posts
 * @author Bill Erickson
 * @link http://wordpress.org/extend/plugins/display-posts-shortcode/
 *
 * @param array $args
 * @param array $atts
 * @return array $args
 */
function be_display_posts_shortcode_offset_posts( $args, $atts ) {
	if( isset( $atts['offset'] ) ) {
		$args['offset'] = (int) $atts['offset'];
	}

	return $args;
}
add_filter( 'display_posts_shortcode_args', 'be_display_posts_shortcode_offset_posts', 10, 2 );