Can you explain a bit more what you are trying to do? Do you want the PDF date field to show the date of the form submission or the current date (when PDF file is opened)? Or user specified date perhaps?
User specified date: https://contactform7.com/date-field/
Submission date: https://contactform7.com/hidden-field/ + some javascript on the web page possibly? Or a special CSS class combined with a JS library.
The link you sent seems to describe how to have the PDF field populate with the current date (of when the PDF file is opened). I haven’t tried it myself but it should work.