[Kirk Y]

There is no difference; vBulletin uses time() as TIMENOW.

See line 32 of init.php:

define('TIMENOW', time());

Check out line 1335 of functions.php:

$user['securitytoken'] = sha1($user['userid'] . sha1($user['salt']) . sha1(COOKIE_SALT));
$user['logouthash'] =& $user['securitytoken'];