If you look at the generated source of a page with comments, you will see something like this
HTML Code:
<a name="comments"></a><!-- named anchor for skip links -->
<h3 id="comments">3 comments to Post with thumbnail and image</h3>
<div class="navigation-comments-above"><a class='prev page-numbers' href='/wordpress/?p=27&cpage=1#comments'>« Older Comments</a>
<a class='page-numbers' href='/wordpress/?p=27&cpage=1#comments'>1</a>
<span class='page-numbers current'>2</span></div>
<!-- Comment List -->
<ul class="commentlist">
<li class="comment byuser comment-author-admin bypostauthor even thread-even depth-1 clearfix" id="comment-12">
<div id="div-comment-12" class="clearfix comment-container bypostauthor">
<div class="comment-author vcard">
<span class="authorname">admin</span>
</div>
<div class="comment-meta commentmetadata">
<a href="http://yourdomain.com/wordpress/?p=27&cpage=2#comment-12">
February 23rd, 2010 at 1:45 pm</a>
<span class="comment-reply-link"><a rel='nofollow' class='comment-reply-link' href='/wordpress/?p=27&cpage=2&replytocom=12#respond' onclick='return addComment.moveForm("comment-12", "12", "respond", "27")'> · Reply</a></span> </div>
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi varius vestibulum rutrum. Quisque in ligula lacus, sed scelerisque nisl. Proin congue sodales leo ac eleifend. Curabitur mollis accumsan varius. Donec a velit risus. Integer egestas, sapien in pellentesque sagittis, odio leo posuere turpis, a ultrices quam est sit amet lacus. Nunc at lectus elit, vitae egestas augue. Maecenas auctor tempus leo, vitae egestas magna ornare sit amet. Morbi sollicitudin convallis scelerisque.</p>
</div>
if you look, you will see the date is part of an anchor '<a...> that is part of a <div..> with a class comment-meta. With this information, you can construct a CSS Selector and the rules you want to apply.