The invoice date is usually the date when you send the invoice. The due date will be the date you and your customer agree on.
The invoice date and the date when the invoice was sent will not necessarily have to be the same date if you agree otherwise. The invoice will always be sent when you send it, regardless whether you have set a future billing date. The record will show actual date when the invoice was sent since Billogram's records use the invoice method.
You can set how many due days you want by default on your invoices in Settings. You can for example choose to have 30 days payment time as standard on your invoices.