I php you can create an anchor class based on the desired date.
I haven’t tested it, but it should be something like this:
// your relevant html parent element
<div class="<?php echo getOlderPostsClassName( '5/5/2012',the_time() ); ?>">
function getOlderPostsClassName($untilDate, $wordpressPostDate)
{
$start = strtotime($untilDate);
$end = strtotime($wordpressPostDate);
if ($start-$end > 0)
{
return 'My_css_class_for_older_posts';
}
else
{
return '';
}
}
Then in your Css you can target differently any descendant element:
.My_css_class_for_older_posts h2{
color:red;
}