SELECT T1.date_order_placed FROM orders AS T1 JOIN shipments AS T2 ON T1.order_id = T2.order_id WHERE T2.invoice_number = 10
What is the placement date of the order whose invoice number is 10?
[Schema (values) (types)]: | tracking_orders |  Customers : customer_id (text) , customer_name (number) , customer_details (text) | Invoices : invoice_number (text) , invoice_date (number) , invoice_details (text) | Orders : order_id (text) , customer_id (number) , order_status (text) , date_order_placed (text) , order_details (number) | Products : product_id (text) , product_name (number) , product_details (text) | Order_Items : order_item_id (text) , product_id (number) , order_id (text) , order_item_status (text) , order_item_details (number) | Shipments : shipment_id (text) , order_id (number) , invoice_number (text) , shipment_tracking_number (text) , shipment_date (number) , other_shipment_details (time) | Shipment_Items : shipment_id (text) , order_item_id (number);