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When a recurring job should use systemd’s service management—or catch up after a calendar event passed while its timer was inactive—I prefer a systemd timer. A timer activates a service unit, so the scheduled work can be managed and inspected like other systemd services. That is a practical advantage, not proof that timers are always better: for a basic, portable schedule, a crontab may be simpler.
What a systemd timer does
A timer unit defines when to activate a service unit; the service unit defines the work. This separation means the schedule and the job have distinct configuration, while the job remains part of systemd’s unit management. The systemd project’s timer manual describes the available timer settings and how they activate units. Oracle Linux also documents systemctl list-timers for inspecting timers and points users to systemd.timer(5) and systemd.time(7) for details: Oracle Linux: Working with Timers in Systemd.
I prefer this arrangement when a scheduled task benefits from systemd service configuration and management. It is especially useful when the schedule is more than a single uncomplicated calendar entry, or when timing should relate to boot or another unit’s activity.
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Choose calendar time or elapsed time
Use OnCalendar= for wall-clock schedules
OnCalendar= expresses realtime calendar events, such as a task scheduled for a particular time of day. Because it depends on the system’s realtime clock, an incorrect clock can lead to unexpected activation times.
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Use monotonic settings for schedules relative to events
Settings such as OnBootSec=, OnStartupSec=, OnActiveSec=, OnUnitActiveSec=, and OnUnitInactiveSec= specify elapsed time relative to manager or unit events. They do not depend on wall-clock time or time zones. The systemd manual permits calendar and monotonic expressions to be combined in a timer unit, so the choice need not be limited to one timing model.
What happens when the computer is off or asleep?
Calendar timers can catch up once
For a calendar timer, Persistent=true records the last-triggered timestamp. When the timer is activated again, it can trigger the service if at least one scheduled event elapsed while the timer was inactive. This is a missed-event catch-up, not a queue that replays every missed occurrence.
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If the computer sleeps, its realtime clock continues. An elapsed calendar event can therefore be handled after resume; if several events pass during continuous sleep, the result is one service activation, not one activation per missed event. A Linux community discussion captures the practical question—“How do you handle tasks that must run even if the PC was turned off?”—but that phrasing is an example of a user concern, not survey evidence: Reddit discussion.
Monotonic timers do not generally wake a suspended machine
Monotonic clocks generally pause during suspend. WakeSystem= can select a clock that continues during suspend, but waking the computer also depends on hardware support. Do not assume that a timer alone can power on a sleeping or powered-off machine.
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Timers allow timing windows, not real-time guarantees
AccuracySec= defines an allowed timing window. The current upstream systemd manual gives a default of one minute: the manager schedules expiration within the interval beginning at the configured expiration and ending after the accuracy interval, at a host-specific stable position. This is a documented default, not a performance statistic or a hard real-time guarantee.
RandomizedDelaySec= has a different role: it adds a random delay to spread timer events and reduce workload spikes. Neither setting should be mistaken for a precision guarantee. A very small accuracy setting does not override system timer slack or turn a timer into a hard real-time scheduler.
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Calendar timers depend on a trustworthy clock
The systemd manual warns that an incorrectly set realtime clock can produce unexpected calendar-timer activation. It documents that timer units with at least one OnCalendar= expression are ordered after time-sync.target, with systemd-time-wait-sync.service ordered before that target. Check that time synchronization is configured appropriately, especially on systems without a reliable battery-backed real-time clock. The cited manual is the systemd project’s moving main branch, so consult the installed systemd.timer(5) documentation for your distribution’s version and defaults.
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When I choose a timer—and when I keep cron
| Consideration | systemd timer | Cron |
|---|---|---|
| Host and portability | A natural fit on a system managed by systemd. | Often the more portable choice across varied Unix-like systems. |
| Schedule semantics | Supports calendar schedules and elapsed-time schedules tied to manager or unit events; calendar timers can use Persistent=true for a missed-event catch-up. |
A crontab can be a straightforward fit for basic recurring calendar schedules. |
| Service management | The scheduled work is a service unit and can use systemd’s service management. | A schedule is expressed in the crontab; the systemd timer/service pairing is not required. |
| Configuration effort | Requires a timer and service unit, which can be worthwhile when their integration is useful. | A single crontab entry may be easier to maintain for a few simple jobs. |
| Timing tolerance | Provides an accuracy window and optional randomized delay, not hard real-time precision. | Choose according to the needs and behavior of the cron implementation in use; the cited sources establish no comparative timing benchmark. |
The portability and simplicity tradeoff is a practical preference reflected in community discussion, not a universal rule or empirical comparison: Reddit discussion: Why I Prefer systemd Timers Over Cron.
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- Choose a systemd timer when the machine uses systemd and the job benefits from a service unit, systemd inspection, event-relative scheduling, or calendar missed-event catch-up.
- Keep cron when the schedule is simple, a crontab is easier for your team to maintain, or portability across different Unix-like systems matters more than systemd integration.
- If a task requires strict execution at an exact instant, do not treat either configuration choice as a hard real-time guarantee; verify that the scheduler and system design meet that requirement.
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