https://github.com/holger1411/understrap/issues/107 - Thx @Thomas-A-Reinert

author.php

@@ -59,7 +59,7 @@
               <li>
                 <a rel="bookmark" href="<?php the_permalink() ?>" title="Permanent Link: <?php the_title(); ?>">
                 <?php the_title(); ?></a>,
-                <?php the_time('d M Y'); ?> in <?php the_category('&');?>
+                <?php understrap_posted_on(); ?> <?php esc_html_e( 'in', 'understrap' ); ?> <?php the_category('&');?>
               </li>
               <?php endwhile; ?>
 

package.json

@@ -43,5 +43,6 @@
     "browser-sync": "^2.17.5",
     "owl.carousel": "^2.2.0",
     "undescores-for-npm": "^1.0.0"
-  }
+  },
+  "devDependencies": {}
 }