Ubuntu 16.04
/etc/systemd/system/certbot.service
[Unit]
Description=Let's Encrypt renewal
[Service]
Type=oneshot
ExecStart=/usr/bin/certbot renew --quiet --agree-tos
ExecStartPost=/bin/systemctl reload apache2.service
/etc/systemd/system/certbot.timer
[Unit]
Description=Twice daily renewal of Let's Encrypt's certificates
[Timer]
OnCalendar=0/12:00:00
RandomizedDelaySec=1h
Persistent=true
[Install]
WantedBy=timers.target
Enable and start certbot.timer
systemctl enable --now certbot.timer
Modified from: https://wiki.archlinux.org/index.php/Let%E2%80%99s_Encrypt#systemd