By default, the invoice date is calculated as the date the job was started. If you (the service company) desire the invoice date to be something other than the date the job/visit occurred, you can configure a different invoice date using the 'Invoice Date' field located to the lower-right on the job entry/edit page below Additional Options. Any value entered here takes priority over the default start date.
The due date is generally calculated using the service's terms of service, or more commonly called terms, selected on the job entry/edit page below Additional Options. For example, Net 30 means that if a job has an invoice date of September 15th, it is expected that the balance for the job will be paid on or before October 15th.