Cookies and timestamp for using last visit

[Terminator]

I have a photogallery site wich uses also can leave reactions on specific photos.

Now I want to create that you see new reactions on the photos.
The situation is now, the users can login to the site using their username + password and a cookie gets set :

$objuserinfo = mysql_fetch_object($resultlogin);
$unix_timestamp = mktime(); 
setcookie("userid", $objuserinfo->m_id, time() + (10 * 365 * 24 * 60 * 60), "/");
setcookie("password", $objuserinfo->m_password, time() + (10 * 365 * 24 * 60 * 60), "/");
setcookie("lastvisit", $unix_timestamp, time() + (10 * 365 * 24 * 60 * 60), "/");

Now I want to create it that when a person leaves the site that the user sets a new lastvisit in the cookie (or do I need to have a DB with that ??).

[RossC0]

You should set it when the user starts at your site because you dont know when they will exit!

All you need to do is check for any reactions that are greater than the last login time before writing the new cookie and vola!