Systemd Timer Errors — Replacing Cron with Systemd Timers and Troubleshooting

Fix systemd timer errors including timers not triggering, incorrect scheduling, and transitioning from cron to systemd timers for better logging and control. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: Systemd timers are the modern replacement for cron, offering better logging via journalctl. Each timer requires two unit files: a .timer file and a corresponding .service file. Timer types: OnCalendar (calendar-based) and OnBootSec/OnUnitActiveSec (relative to events). Timers are managed with systemctl: enable, start, status, list-timers. AccuracySec and RandomizedDelaySec prevent all timers from firing at the exact same time. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Timer not enabled: created but not started with systemctl enable --now. Corresponding .service file missing or misconfigured. OnCalendar expression syntax incorrect (use systemd-analyze calendar to validate). Timer enabled but service unit fails silently — not checked via journalctl. AccuracySec too large causing timer to fire minutes after the intended time. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: List active timers: systemctl list-timers --all. Create timer: /etc/systemd/system/mytask.timer with [Timer] section containing OnCalendar=. Create service: /etc/systemd/system/mytask.service with [Service] section containing ExecStart=. Validate calendar expression: systemd-analyze calendar 'Mon..Fri *-*-* 09:00:00'. Enable and start: systemctl daemon-reload && systemctl enable --now mytask.timer. Check logs: journalctl -u mytask.service to see output from the timer-triggered service. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

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