Logout all users at particular time

@C0c0b33f nearly has the correct answer, but to log out all users, it is necessary to use the WP_Session_Tokens class. If you change the myplugin_cron_function in the previous answer to the following it should work.

// hook function myplugin_cron_hook() to the action myplugin_cron_hook.
add_action( 'myplugin_cron_hook', 'myplugin_cron_function' );
/**
 * Create hook and schedule cron job on plugin activation.
 * Schedule recurring cron event.
 * hourly
 * twicedaily
 * daily
 */
function myplugin_activate() {
    $timestamp = strtotime( '24:00:00' ); // 12:00 AM.
    // check to make sure it's not already scheduled.
    if ( ! wp_next_scheduled( 'myplugin_cron_hook' ) ) {
        wp_schedule_event( $timestamp, 'daily', 'myplugin_cron_hook' );
        // use wp_schedule_single_event function for non-recurring.
    }
}
register_activation_hook( __FILE__, 'myplugin_activate' );


/**
 * Unset cron event on plugin deactivation.
 */
function myplugin_deactivate() {
    wp_clear_scheduled_hook( 'myplugin_cron_hook' ); // unschedule event.
}
register_deactivation_hook( __FILE__, 'myplugin_deactivate' );

/**
 * Function called by the cron event.
 */
function myplugin_cron_function() {
    // Get an instance of WP_User_Meta_Session_Tokens
    $sessions_manager = WP_Session_Tokens::get_instance();
    // Remove all the session data for all users.
    $sessions_manager->drop_sessions();
}

You could use WP Cron to fire wp_logout() and make it more reliable by following instructions here https://developer.wordpress.org/plugins/cron/hooking-wp-cron-into-the-system-task-scheduler/

Below is code for a plugin that would log everyone out, every day at midnight. Note if you wanted to schedule a one-time-only, single event, use wp_schedule_single_event instead of wp_schedule_event https://codex.wordpress.org/Function_Reference/wp_schedule_single_event

// hook function myplugin_cron_hook() to the action myplugin_cron_hook.
add_action( 'myplugin_cron_hook', 'myplugin_cron_function' );
/**
 * Create hook and schedule cron job on plugin activation.
 * Schedule recurring cron event.
 * hourly
 * twicedaily
 * daily
 */
function myplugin_activate() {
    $timestamp = strtotime( '24:00:00' ); // 12:00 AM.
    // check to make sure it's not already scheduled.
    if ( ! wp_next_scheduled( 'myplugin_cron_hook' ) ) {
        wp_schedule_event( $timestamp, 'daily', 'myplugin_cron_hook' );
        // use wp_schedule_single_event function for non-recurring.
    }
}
register_activation_hook( __FILE__, 'myplugin_activate' );


/**
 * Unset cron event on plugin deactivation.
 */
function myplugin_deactivate() {
    wp_clear_scheduled_hook( 'myplugin_cron_hook' ); // unschedule event.
}
register_deactivation_hook( __FILE__, 'myplugin_deactivate' );

/**
 * Function called by the cron event.
 */
function myplugin_cron_function() {
    // error_log( print_r( 'yes we have wp-cron', true ) );
    wp_logout();
}

/**
 * View all currently scheduled tasks utility.
 */
function myplugin_print_tasks() {
    echo '<pre>';
    print_r( _get_cron_array() );
    echo '</pre>';
}