[B]DELETE FROM wp_posts WHERE post_date < '2008-06-06 19:18:00' AND post_status = 'publish'
worked for me thanksif your comfortable with phpmyadmin you can do this make sure to backup before do this and change date to desired date (This SQL query will delete the wordpress posts published before June 6, 2008.)
Code:[B]DELETE FROM wp_posts WHERE post_date < '2008-06-06 19:18:00' AND post_status = 'publish'[/B]
if not try this WordPress ? Bulk Delete « WordPress Plugins