There are a couple of ways you can do this. If you would just like to add in the current date this would be the simplest thing to do. You could add in the current date in php using a timestamp and the date function to format it like this:
<?php echo date("your date format string", time()) ?>
Here is the link to the date function page on php if you are interested in doing it this way:
If you would like to get this value from your db you will need to add a recordset to the email page and filter it on the session variable that holds your order id. This variable is part of the 'store order summary' server behavior that is normally on the confirm page. With a recordset that is filtered on this value you could make use of the bindings to add in information about the order to the email including the date value that is held in the table.