public function setBirthday( $value )
	{
		if ( $value === $this->getBirthday() ) { return; }

		if( $value !== null )
		{
			$this->_checkDateOnlyFormat( $value );
			$value = (string) $value;
		}

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