// Detect if post is in the past and if so add a custom Class "oldpost"
if (strtotime(get_the_date( "Y-m-d", $the_id )) < strtotime(date("Y-m-d"))) {
 $post_class = "post-entry post-entry-{$the_id} oldpost slide-entry-overview slide-loop-{$post_loop_count} slide-parity-{$parity} {$last}";
 }
else {
 $post_class = "post-entry post-entry-{$the_id} slide-entry-overview slide-loop-{$post_loop_count} slide-parity-{$parity} {$last}";
 }