Source — /etc/systemd/system/timers.target.wants/apt-daily.timer

/etc/systemd/system/timers.target.wants/apt-daily.timer

[Unit]
Description=Daily apt download activities

[Timer]
OnCalendar=*-*-* 6,18:00
RandomizedDelaySec=12h
Persistent=true

[Install]
WantedBy=timers.target