Why does Google not recognize hentry updated?

Question

I have the following code for my date in my blogpost:

<time class="date updated" datetime="2015-10-06">
<a href="http://www.foo.de/2015/10/06/">6. Oktober 2015</a>
</time>

But the Google Structured Data Testing Tool does not recognize the datetime. What am I doing wrong?

Here is the PHP version:

<time class="date updated" datetime="<?php the_time('Y-m-d'); ?>">
        <a href="<?php echo get_day_link( get_the_time('Y'),
        get_the_time('m'), get_the_time('d')); ?>">
        <?php echo get_the_date() ?>
        </a>
</time>

Here comes more: This is the part in single.php where the Date and other metadata gets loaded.

<header class="topinfo animated" data-anim="fadeInDown">
    <h1 title="<?php the_title() ?>" class="entry-title"><?php the_title() ?></h1>
    <?php get_template_part('part', 'textinfo') ?>
</header>

This is the file where the date and the metadata come from

<?php if( ( !is_page() and $iw_opt['single-date'] ) or ( is_page() and $iw_opt['page-date'] ) ) : ?>
    <li>
        <time class="date updated" datetime="<?php the_time('Y-m-d'); ?>">
        <a href="<?php echo get_day_link( get_the_time('Y'), get_the_time('m'), get_the_time('d')); ?>">
        <?php echo get_the_date() ?>
        </a>
        </time>
    </li>
<?php endif; ?>