You can change 'invoice date' before you validate an invoice

or try this on code
add this line in 'def _prepare_invoice' function>>invoice_vals ={}

ie in sale.py file line no between 237-250

'date_invoice': order.order_date.date()

Hello Noah,

Your question was the same as my question, but I currently use Odoo 11. I'm grateful for having found the answer from this thread, so I want to share my solution (for Odoo 11.0) instead of creating a new thread.

What I changed in the code is the confirmation_date value, I see that if I change the confirmation_date, the generated stock picking scheduled date will also have the same value, and that is what I need in my case. Then the confirmation date value will be assigned to invoice date as well when I create an invoice based on that sales order. The line affected in the code file is https://github.com/odoo/odoo/blob/11.0/addons/sale/models/sale.py#L523

I changed the code from

'confirmation_date': fields.Datetime.now()

into

'confirmation_date': self.date_order

that's all.