Crontab is the obvious choice.
I'm a little curious why you need to shut down the site database at all. If you use dates with your orders you should be able to limit the orders by date, thus any order placed after your "magic" time limit, is excluded from the selection...
If you absolutely must stop entries into the database, then perhaps you should simply hotswap to standby database and still avoid the downtime. I don't have an in depth understanding of your model, but I suspect there is room for improvement.
Confucius says kill mosquito unless cannon