With Celestial, you will get the published (created) date on the blog summary view, but also the full post view. Then as for your modified date, this shows below the post (providing you update the post). But if you want this in the post summary as well next to the published date, you will need to modify the content.php file (found in the folder “partials” with the code from the content-single.php file there:
<?php _e( 'Last Modified: ', 'st' ); ?>
You will probably have to edit the styles for it, but that can be done with doing custom CSS.