The access-date parameter is for "Full date when the content pointed to by url was last verified to support the text in the article;". I am not even given the option to review to ensure it's correct.
I was attempting to use the bot for dead links here: https://en.wikipedia.org/w/index.php?diff=prev&oldid=938795928 and it left access-dates.
IABot adds access dates by attempting to assess when the URL was last added to the article. It's usually very accurate, but inaccuracies can be as a result of URLs being removed and readded or someone modifying the access time in IABot's DB. The bot performs a binary scan on the articles' history to find this information. For the rare cases where it's wrong would require more development time than the amount of benefit gained from it. If you see a persistent pattern of bad access dates. Please re-report.