Invoice Date and Format
There are a few different layouts and date formats available for invoices and statements.
To pick a format go to Admin -> Billing -> Company Options and click the Formatting tab.
Select your preferred date layout with the Invoice and Statement Date Format setting.
If you would like to have your fulfillment order invoices use the order date instead of the current date when invoices are generated, you can turn on the setting labeled Invoice Using Order Date Rather Than Current Date.