dtstart warning on UserComments schema

I have the following code for rich snippet on comments:

<ul itemscope itemtype="http://schema.org/UserComments">
    <li id="comment-1" class="comment">
        <span itemprop="name" class="author">Author 1</span>
        <p itemprop="commentText">Bla Bla Bla</p>
        <time itemprop="commentTime" content="2012-07-29" datetime="2012-07-29T05:55+00:00" title="Jul 29, 2012 5:55">2 days ago</time>
    </li>

    <li id="comment-2" class="comment">
        <span itemprop="name" class="author">Author 2</span>
        <p itemprop="commentText">yada yada yada</p>
        <time itemprop="commentTime" content="2012-07-30" datetime="2012-07-30T04:44+00:00" title="Jul 30, 2012 4:44">yesterday</time>
    </li>
 </ul>

According to schema.org/UserComments, this is correct. However, Google's Rich Snippets Testing Tool is giving an warning:

Warning: Missing required field "dtstart".

dtstart is not even a property of UserComments event. Should I ignore this warning (Google's tool is beta)? Or am I missing something?