add_filter( 'wf_fedex_estimated_delivery',  'wf_add_delivery_time', 15, 20 );
function wf_add_delivery_time( $est_delivery_html, $est_delivery ) {
    $days_to_add = 1;
    if( isset($est_delivery['fedex_delivery_time']) ){
        $new_date = strtotime( "+$days_to_add day", strtotime($est_delivery['fedex_delivery_time']) );
        $new_date = date(  apply_filters('wf_estimate_delivery_date_format', 'd-m-Y' ), $new_date );
        $est_delivery_html = "<br /><small>".__('Est delivery: ', 'wf-shipping-fedex'). $new_date.'</small>';
    }
    return $est_delivery_html;
}