How to modify Author Pane date fields?
Author Pane module contains appropriate templates files which control output. 
author-pane.tpl.php is something that will help. 

Just copy it in your theme directory, flush cache and do whatever you like with this file to customize Auth Pane block.

To remove 'Last seen:' from the pane comment or remove from the file  

&lt;?php /* Last active */ ?&gt;
&lt;?php if (!empty($last_active)): ?&gt;
    &lt;div class="author-pane-line author-last-seen"&gt;
       &lt;span class="author-pane-label"&gt;&lt;?php print t('Last seen'); ?&gt;:&lt;/span&gt; &lt;?php print t('!time ago', array('!time' =&gt; $last_active)); ?&gt;
    &lt;/div&gt;
&lt;?php endif; ?&gt;


To shorten the display format of 'Join' find appropriate admin page (/admin/config/regional/date-time) and change Short date format to the format you prefer.

 * - $joined: Date the post author joined the site. (Uses shortdate format)