>but I assume that only works/displays when the shop is closed?
no, it also works when the shop is open. (as you could of course pre-order for in about 2 hours time or something)
you can have a play with it here to see how it works
https://www.wp-pizza.com/twentytwelve/
(it will be available/shown on the order page , so just put stuff into the cart and go to the order page where you will se a “when would you like the order for” text field….
as ever, any questions etc , let me know