Turn on Sell when out of stock for a variant on Price and inventory to take back orders or pre-orders. An optional Expected to ship date is shown on the product page, in the cart and checkout, and on the order and its emails. Stock goes below zero as these items sell, and payment is taken when the customer orders. Orders has a Pre-orders tab listing orders with items that were not in stock when they were placed.
Pre-orders and back orders in Store
A variant can keep selling when it is out of stock, with an expected ship date, and such orders have their own Pre-orders tab.