I just converted to this theme with an older website and had used various snippets of code (which I found in the support forum) to show the last updated date on posts and pages. Unfortunately, even those these work on the site, Google instead pulled in the published date. Having dates from 2014 show up in the SERPS is obviously not ideal so I removed all of the code snippets and turned off the dates via the theme customizer.
I believe this has successfully removed the old published dates removed from Google results, I am now attempting to get Google to show the last modified date using this plugin https://wordpress.org/plugins/wp-last-modified-info/ which has worked well for me with other themes in the past.
I am just hoping you can assist me with the best configuration of the plugin for your theme.
Under the schema tab in the plugin settings, I am given these options: