Private vairable not working

On the top above my __construct() area in my library I have a variable called $set_the_time_for_cookie_to_expire because the time() has brackets it is throwing error

Code:

A PHP Error was encountered

Severity: Parsing Error

Message: syntax error, unexpected '(', expecting ',' or ';'

Filename: libraries/Customer.php

Line Number: 6

Backtrace:

Is there any way to be able to use php time() like what I am after?

PHP Code:

private $customer_id;
private $set_the_time_for_cookie_to_expire = time() + 20 * 60;
    
public function __construct() {
   $this->CI = &get_instance();
   $this->CI->load->library('session');

   if (!$this->CI->session->userdata('is_logged_in')) {
      $this->autologin();
   }

} 


There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!