Update: Genesis now includes 2 short codes for post modified date and post modified time:
You can use one or both in the Genesis Simple Edits plugin or code like this in your functions file:
This code creates a new shortcode which you can add to your post info or post meta field in Genesis.
Please copy ALL the PHP code from the view raw link and paste it at the end of your child themes functions.php file using a text editor like Notepad++
You can now add the shortcode named modified_date using this custom function.