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<?php if ( (time() - 24*60*60) < strtotime($topic->topic_time) ) : ?>*NEW* <?php endif; ?>

A very rough way of doing it there, but if you put that in forum.php before say, <?php topic_title(); ?> if you want it in the topic title, it should work. Change 24*60*60 to the number of seconds you want to include topics from.