$customers = Mage::getModel('customer/customer')->getCollection()
                                                        ->addAttributeToSelect('*')
                                                        //->group('postcode')
                                                        ->addFieldToFilter('created_at', array(
                                                            'from' => $this->getStartDate(),
                                                            'to' => $this->getEndDate(),
                                                            'date' => true));