There is no guide, but i’ll try to help you.
Open up application/views/invoice_templates/pdf/InvoicePlane.php, there is a chance you’ll have to add your date in this file.
The date comes from the variable $item
You’ll have to print_r() that $item to see what’s in there. All fields from the table are in there, so it will be something like $item['item_date]
From what I can tell you need an understanding of PHP to make sense of any “solutions” out there. No one has bothered to post a simple to follow step by step guide that explains the code and where to put it.
I’ve put it on pause for the time being until I have time to dig deeper for a solution.