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# systemctl cat 'apt-daily*.timer'
# /lib/systemd/system/apt-daily.timer
[Unit]
Description=Daily apt download activities

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

[Install]
WantedBy=timers.target

# /etc/systemd/system/apt-daily.timer.d/override.conf
### /lib/systemd/system/apt-daily.timer
[Timer]
OnCalendar=
OnCalendar=*-*-* 2,14:15
RandomizedDelaySec=30min

# /lib/systemd/system/apt-daily-upgrade.timer
[Unit]
Description=Daily apt upgrade and clean activities
After=apt-daily.timer

[Timer]
OnCalendar=*-*-* 6:00
RandomizedDelaySec=60m
Persistent=true

[Install]
WantedBy=timers.target

# /etc/systemd/system/apt-daily-upgrade.timer.d/override.conf
## /lib/systemd/system/apt-daily-upgrade.timer
[Timer]
OnCalendar=
OnCalendar=*-*-* 3,15:00
RandomizedDelaySec=30m