It's probably the fun and games with quotes after the WHERE date_purchased part. Let's use some syntax highlighting (free with this forum and any PHP distribution) and see...
$query = 'SELECT `customers_name` , `customers_street_address` , `customers_city` , `customers_postcode` , `customers_state` , `customers_telephone` , `customers_email_address` , `date_purchased` '
. ' FROM `orders` '
. ' WHERE `date_purchased` > '".$POST['start_year']."-".$POST['start_month']."-".$POST['start_day']." 00:00:00' AND `date_purchased` < '".$POST['end_year']."-".$POST['end_month']."-".$POST['end_day']." 00:00:00' LIMIT 0, 30';
And I would appear to be right.