example sitges web//modules/custom/sff_warnings
https://befused.com/drupal/ultimate-cron/
ejemplo en smsjd_drupal omg_ga_reporting.module
ejemplo sitges sff_warning module
1- install ultimate_cron -> activate
2- in custom module /file.module 
  crear 
   function custom_module_cron() {
    \Drupal::logger('custom_utility')->notice('Cron ran');
  }
3- Hit the Discover jobs button and the job you created above will appear on the list.
4- copy configuration localhost:10162/es/admin/config/development/configuration/single/export

  trabajo del cron and nombre que has creado in module 
  pega en imprendincible  que el file en config llamma ultimate_cron.job.unnombrequeqieres.yml  
  config/utility_cron.job.nombre.yml like this 
   langcode: ca
status: true
dependencies:
  module:
    - sff_warnings
title: 'Publish and Unpublish warnings based on dates'
id: sff_warnings_publish_warnings_cron
weight: 0
module: sff_warnings
callback: sff_warnings_publish_warnings_cron
scheduler:
  id: simple
launcher:
  id: serial
logger:
  id: database
in module elimina el function
crear otra del nombre callback like sff_warnings_publish_warnings_cron and put you logica
importa configuracion con comando druash
drush config-import --source=modules/custom/custom_utility/config --partial -y