systemd timers as a cron replacement: OnCalendar, Persistent= and RandomizedDelaySec

Goal

Schedule a recurring job with a systemd timer instead of a crontab entry, so that a missed run (host was off) can be caught up and load can be spread across many hosts.

Prerequisites

Root access on a systemd-based distribution; a command to run, wrapped as an idempotent script.

Steps

1. Write the job as a oneshot service, /etc/systemd/system/backup.service:

   [Unit]
   Description=Nightly backup
   
   [Service]
   Type=oneshot
   ExecStart=/usr/local/bin/backup.sh
   

2. Write the matching timer, /etc/systemd/system/backup.timer:

   [Unit]
   Description=Run backup.service nightly
   
   [Timer]
   OnCalendar=*-*-* 03:00:00
   Persistent=true
   RandomizedDelaySec=15min
   
   [Install]
   WantedBy=timers.target
   

   OnCalendar= uses the calendar event syntax described in systemd.time(7) (weekday names, ranges and * wildcards for date fields). Persistent=true makes systemd run the service once, on the next boot or timer activation, if the last scheduled run was missed while the system was off. RandomizedDelaySec= adds a uniformly distributed random delay up to the given value on top of OnCalendar=, useful to avoid many hosts hitting a shared resource at the same second.
3. Before enabling, validate and preview the expression: systemd-analyze calendar 'Mon..Fri *-*-* 03:00:00'. It prints the normalized form and the next elapse time without touching any unit.
4. Reload unit files, then enable the timer (not the service) so it survives reboots and starts now: systemctl daemon-reload && systemctl enable --now backup.timer. Run the job once by hand (systemctl start backup.service) to see failures immediately instead of at 03:00.
5. List scheduled and next runs: systemctl list-timers backup.timer (or --all for inactive ones too), showing the NEXT and LEFT columns.

Expected result

systemctl list-timers shows backup.timer with a future NEXT time; journalctl -u backup.service shows a completed run at or after that time (allowing for the random delay).

Limits and test basis

Based on systemd.timer(5), systemd.time(7) and systemd-analyze(1). To undo: systemctl disable --now backup.timer, remove both unit files, then systemctl daemon-reload. Timer accuracy defaults to about one minute (AccuracySec=1min), so sub-minute precision is not guaranteed.