You can define whether the invoice date can be set manually when you create an invoice or automatically when you save it as final.
On the Company tab, click Invoice Types, and then on the Invoice Types page, click Edit next to the appropriate invoice type.
In the Invoice Type dialog box, select the appropriate option in the Date Mode box, and then click OK.
If the Create date mode is selected, you can set the invoice date manually when you create an invoice, if the Final date mode is selected, the invoice date is set automatically when you save the invoice as final.