How to show “Date Last Modified” in posts?

I’ve edited my child theme, but can’t seem to show the Date Last Modified of the post. What I want to happen is that below the post’s title (where the Date, Author’s Name, and Category are located), I wanted to add “Date the post was last modified”.

I’ve edited post-templates/content.php by editing the line:

<time class=”detail left index-post-date” datetime=”<?php echo get_the_date( ‘Y-m-d’ ); ?>”><?php echo get_the_date( ‘M d, Y’ ); ?></time>

but I can’t seem to make it work. Please help how to show the “Date Last Modified” for single posts.

Thank you!