What happens to an order placed one minute before I close?

It is accepted, because it arrived inside your working hours. Outside your hours the page shows your closed screen with a countdown, and even a cart built earlier is refused at checkout with a message saying you are closed. The clock is your restaurant's own time zone. If you want a buffer, end your working hours under Settings → Working hours a little before the kitchen really closes. The Getting started tab in the docs covers working hours.