Can’t change the invoice date.

Hello ??

I’d like to change the invoice date so I wrote the function according to the FAQ, “How to change the invoice date?” (https://www.ads-software.com/plugins/woocommerce-pdf-invoices/#how%20to%20change%20the%20invoice%20date%3F） on functions.php but it doesn’t work, even there is no error.
I want to set it the same date as the order date.
How can I figure it out?

Thanks,