Check out Sources/Subs.php
writelog function
// Set their login time, if not already done within the last minute.
if (SMF != 'SSI' && !empty($user_info['last_login']) && $user_info['last_login'] < time() - 60)
{
// Don't count longer than 15 minutes.
if (time() - $_SESSION['timeOnlineUpdated'] > 60 * 15)
$_SESSION['timeOnlineUpdated'] = time();
$user_settings['total_time_logged_in'] += time() - $_SESSION['timeOnlineUpdated'];
updateMemberData($user_info['id'], array('last_login' => time(), 'member_ip' => $user_info['ip'], 'member_ip2' => $_SERVER['BAN_CHECK_IP'], 'total_time_logged_in' => $user_settings['total_time_logged_in']));