UPS-XML, when gathering the estimated arrival dates, uses the following condition to determine the ship-date submitted to UPS:
PHP Code:
        if (defined('SHIPPING_DAYS_DELAY')) {
             $shipdate = date('Ymd', time() + (86400 * SHIPPING_DAYS_DELAY));
        } else {
            $shipdate = $this->today;
        } 
... where 86400 represents a day's worth of seconds (60 * 60 * 24). If you enable the shipping-method's debug, you'll see the request/response to UPS' TimeInTransit API with the request's PickupDate field identifying the date the shipment's to be made.