public function setDateBack( $backdate )
	{
		if ( $backdate === $this->getDateBack() ) { return; }

		$this->_checkDateFormat($backdate);

		if ( $backdate !== null ) {
			$backdate = (string) $backdate;
		}

		$this->_values['backdate'] = $backdate;
		$this->setModified();
	}