From http://www.webtechideas.com/how-to-get-last-login-time-in-wordpress/

Wordpress User Last Login

<?php //associating a function to login hook add_action('wp_login', 'set_last_login');   //function for setting the last login function set_last_login($login) {    $user = get_userdatabylogin($login);      //add or update the last login value for logged in user    update_usermeta( $user->ID, 'last_login', current_time('mysql') ); } ?>