Joomla version 3.3.6
Using mod_tags_similar, the helper returns items marked as published using core_state=1 but it also returns items which have expired as no check is made on the core_publish_up or core_publish_down columns

Tested by editing the article published state and publish up and down dates.

I fixed it by editing mod_tags_similar/helper.php

added at line 42

$now         = JFactory::getDate()->toSql();
$nullDate    = $db->getNullDate();

and then modified the published tags where query

// Only return published tags
$query->where($db->quoteName('cc.core_state') . ' = 1 ')
     ->where('(' .$db->quoteName('cc.core_publish_up') . '=' . $db->quote($nullDate) . ' OR ' .$db->quoteName('cc.core_publish_up') . '<=' . $db->quote($now) . ')')
     ->where('(' . $db->quoteName('cc.core_publish_down') . '=' . $db->quote($nullDate) . ' OR ' .$db->quoteName('cc.core_publish_down') . '>=' . $db->quote($now) . ')');