systemctl --all list-timers
 
  systemctl stop    certbot.timer
  systemctl disable certbot.timer
  systemctl mask    certbot.timer
 
  m=$(( RANDOM % 60 )); h=$(( RANDOM % 24 )); d=$(( RANDOM % 7 ))
  # dom=$(( 1+ RANDOM % 31 )) mon=$(( 1+ RANDOM % 12 ))
  echo "## Let's Encrypt SSL certificate renewal with certbot" | tee -a /etc/crontab
  echo "$m $h * * $d root /usr/bin/certbot renew"              | tee -a /etc/crontab