Automatically delete inactive users after 2 months

I am trying to modify this function:

// automatically delete users after 7 days in wordpress
function wcs_auto_delete_users() {
global $wpdb;
$query = $wpdb->prepare( "SELECT ID FROM $wpdb->users WHERE datediff( now(), user_registered ) > 7" );
if ( $oldUsers = $wpdb->get_results( $query, ARRAY_N ) ) {
    foreach ( $oldUsers as $user_id ) {
        wp_delete_user( $user_id[0] );
    }
}
}
add_action( 'wcs_daily_clean_database', 'wcs_auto_delete_users' );

wp_schedule_event( time(), 'daily', 'wcs_daily_clean_database' );

to work differently instead — I want it automatically delete users who haven't been active in, say, 2 months. I have a plugin that tracks user activity and stores the data in wp_usermeta. Example:

user_id = 2; meta_key = wp_wp_kc_last_active_time; meta_value = 1422796627

This is the query that I've come up with:

SELECT user_id FROM wp_usermeta WHERE meta_key = 'wp_wp_kc_last_active_time' AND TIMESTAMPDIFF( second, now(), TIMESTAMP(SELECT meta_value) ) > 5184000

But it's not selecting the right IDs. What should I change to make it work?