Thanks for reaching out to the Community, loilars.

If an invoices Ship Date field is left blank, it will be defaulted to your invoice's date. In the event you'd prefer not to enter shipping dates on invoices, you can customize their templates and remove your Ship Date field.

Here's how:
 

1. Open the invoice.
2. Access your Formatting tab.
3. Click Customize Data Layout.
4. On the Header screen, find your Ship Date option and untick its Screen and Print checkboxes.
5. Select OK.

I've also included a detailed resource about working with sales form templates which may come in handy moving forward: Use & customize form templates

Please don't hesitate to send a reply if there's any additional questions. Have an awesome Wednesday!