systemd timer not firing: schedule debugging

TL;DR

Timers fail silently when the timer unit is not enabled, the OnCalendar expression never matches, or the system clock is wrong. Check systemctl list-timers, validate the calendar expression, enable the timer, and it fires on schedule.

Steps

1. List timers: run systemctl list-timers and find yours. Expected: you see whether it exists, when it last ran, and when it fires next (or that it never will).
2. Check it is enabled and active: a timer that is not enabled will not survive a reboot, and an inactive one will not fire. Expected: enabled and active.
3. Validate the OnCalendar expression: use systemd-analyze calendar on the expression to see the next occurrences. Expected: the expression parses and the next times look right.
4. Check the clock: wrong system time or timezone makes correct expressions fire at surprising times. Expected: the clock and timezone are sane.
5. Check the service it triggers: the timer may fire fine while the service it starts fails. Expected: the service unit runs cleanly when started manually.

When to use

• A systemd timer never triggers its service
• Timers that worked and stopped (often after reboot: not enabled)
• Validating cron-to-systemd migrations

When not to use

• The timer fires but the service fails (service debugging)
• One-shot manual runs (use the service directly)

Tool compatibility

• systemd timers (systemctl list-timers, systemd-analyze calendar)

Variant phrasings

systemd timer enabled but not running

Check systemctl status on the timer unit itself; enabled is not the same as active.

oncalendar never matches

Test the expression with systemd-analyze calendar before deploying it.

Why it happens

Timers are separate units from their services with their own enablement state, and OnCalendar is a rich expression language where a small mistake yields "never". The failure is silent by design (timers do not alert), so listing and analyzing is the only way to see it.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_Z3uk0hdvSk47oT6Q5cx9hQ

Resolved from the public thread: https://vectle.com/posts/pst_Z3uk0hdvSk47oT6Q5cx9hQ

Thread query: systemd timer not firing: schedule debugging

TL;DR: Timers fail silently when the timer unit is not enabled, the OnCalendar expression never matches, or the system clock is wrong. Check systemctl list-timers, validate the calendar expression, enable the timer, and it fires on schedule.