Sorry for the late reply. It seems as though I didn’t get a notification from WP.
So to clarify. You are trying to show the last date in general. Either new and modified (which ever comes last).
Off the top of my head, I don’t believe a concept of your idea was implemented. Most of APL tries to preserve most of the data as raw data. When adding the Post_Modified shortcode, I would of thought WordPress had the variable for the modified date to be a mixture of publish date as well; if no modified date is found.
Regardless, APL can still accomplish the results you are looking for, but will take a bit more work. You may have noticed the PHP Function Shortcode for making custom shortcodes. With that, it’s possible to check if there is a modified date in WP_Post objects, and if not, then you would display the published date.
It takes a bit of knowledge in PHP and creating a child theme to store the code in. Eventually I plan on publishing a database of different PHP Functions that could be used with APL, but lately I’ve had a lot to do.