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A cron expression tells a scheduler when to try to run a command; it does not prove the command started, finished, or succeeded. The familiar crontab format has five time fields followed by a command, but parsing rules, time zones, missed runs, and overlap behavior depend on the scheduler. Treat the schedule as one part of a complete, observable task.
What does a cron expression mean?
In the conventional crontab format documented by Cronie, a line contains five schedule fields followed by the command to run. A system crontab may include a user field between the schedule and command.
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| Field | Meaning | Typical values |
|---|---|---|
| Minute | Minute of the hour | 0–59 |
| Hour | Hour of the day | 0–23 |
| Day of month | Calendar day | 1–31 |
| Month | Month | 1–12 |
| Day of week | Weekday | 0–7 or names, depending on implementation |
| Command | Program or script the scheduler attempts to run | Implementation-specific |
For example, 0 3 * * 1 means 03:00 every Monday in the five-field syntax documented for Kubernetes. The expression does not specify a time zone: that must be established by the scheduler or its configuration.
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Cronie documents that the minute, hour, and month fields must match, while at least one of the day-of-month and day-of-week fields must match. In other words, when both day fields are restricted, Cronie uses an OR condition between them. Other schedulers may differ in semantics or accepted extensions, so validate an expression against the exact scheduler reference before deploying it.
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How do I run a cron job every day?
In a five-field crontab, 0 3 * * * requests a run at minute 0 of hour 3 on every day, month, and weekday. It means 03:00 in the scheduler’s configured time zone, not automatically 03:00 wherever the person reading the line happens to be.
Choose the intended clock before writing the schedule. A report meant to arrive at a staff member’s local morning may need to follow a regional time zone; a global cleanup or batch process may be easier to reason about in a stable zone such as UTC. Then confirm how that scheduler handles daylight-saving changes and delayed starts.
Does cron use my local time?
There is no universal answer: time-zone behavior belongs to the scheduler and its configuration. In Cronie, schedules use local wall-clock time; its documentation says a job scheduled in the missing spring-forward hour does not match, while a job scheduled during the repeated fall-back hour may match twice. A local-time schedule can therefore skip or repeat a run around daylight-saving transitions.
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Kubernetes CronJobs
Kubernetes uses the controller manager’s local time zone if .spec.timeZone is omitted. A CronJob can instead name a valid zone such as Etc/UTC. Kubernetes documents that if the configured time-zone name becomes invalid during the resource’s lifetime, the controller stops creating new Jobs and emits an UnknownTimeZone event. See the Kubernetes CronJob documentation and CronJob API reference.
GitHub Actions schedules
GitHub Actions scheduled workflows use UTC by default and can specify an IANA time zone. Its documentation says a workflow scheduled in the skipped spring-forward hour advances to the next valid time; the documented example moves 2:30 a.m. to 3:00 a.m. These are GitHub Actions-specific rules, not general cron behavior. See GitHub’s schedule event documentation.
Why didn’t my cron job run?
Work through the full chain rather than assuming the expression is the only possible cause:
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- Check that the expression is valid for this scheduler. Confirm its field count, day matching, supported syntax, and configured time zone in the scheduler’s own documentation.
- Check whether the scheduled instant occurred. Convert the intended time to the scheduler’s zone and account for daylight-saving transitions where applicable.
- Check whether the scheduler created or launched a run. A missed or delayed occurrence may be skipped under platform-specific rules. Kubernetes, for example, allows
startingDeadlineSecondsto limit how late a missed Job may start; if an occurrence is beyond that deadline, it is skipped while later schedules remain in effect. - Check whether the process started and what it did. Inspect the task’s logs and exit status, not just the schedule definition.
- Check whether the result was visible. A completed process can still fail to produce the expected outcome; use an application-level success signal appropriate to the work.
In Kubernetes, CronJob status exposes active Jobs, the last schedule time, and the last successful time. Those records aid diagnosis but do not by themselves alert someone when a run is missing or unsuccessful.
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That depends on the scheduler and its overlap policy. If a new run starts while an earlier one is still active, the tasks may compete over files, database rows, or external services. Decide whether overlap is safe for the operation instead of treating it as a purely technical preference.
Kubernetes overlap controls
Kubernetes CronJobs support three concurrencyPolicy values. The documented default is Allow; Forbid skips a new scheduled run while a prior Job from that CronJob is active; and Replace replaces an active Job with a new one. Skipping or replacing work may be worse than allowing overlap, depending on whether completing every occurrence matters more than avoiding simultaneous execution.
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Kubernetes also warns that a CronJob can create multiple concurrent Jobs in some circumstances. If duplicate execution could cause damage, make the task safe to repeat—for example, through idempotent operations, a lock, or another application-level guard appropriate to the workload. That is a practical safeguard against the documented limitation, not a guarantee that one specific mechanism is required.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do the major scheduling options differ?
Local cron, Kubernetes CronJobs, and GitHub Actions all support recurring schedules, but they run in different places and carry different operational assumptions. The table summarizes only behavior established in their documentation; it is not a complete feature or pricing comparison.
| Option | Where it runs and code source | Time zone and cadence | Missed runs and overlap | History and visibility |
|---|---|---|---|---|
| Local crontab (Cronie behavior cited) | On the host running the cron service; command is specified in the crontab | Local wall-clock behavior documented by Cronie; five schedule fields | Cronie documents spring-forward skips and fall-back repeats for affected wall-clock times; other missed-run and overlap rules are not stated here | Not stated in the cited Cronie documentation |
| Kubernetes CronJob | Kubernetes controller creates Jobs from a CronJob resource’s job template | Controller manager local time zone unless .spec.timeZone is set; five-field syntax |
startingDeadlineSeconds can cause late missed occurrences to be skipped; policy is Allow, Forbid, or Replace; controller limitations can result in multiple concurrent Jobs |
Status includes active Jobs, last schedule time, and last successful time; successful and failed Job history limits default to 3 and 1 respectively |
| GitHub Actions scheduled workflow | Runs a workflow in GitHub Actions using the latest commit on the repository’s default branch | UTC by default, with optional IANA time zone; documented minimum interval is five minutes | Spring-forward skipped-hour schedules advance to the next valid time; other missed-run and overlap behavior is not stated here | Workflow runs are visible in GitHub Actions; the cited schedule documentation does not establish a full missed-run alerting system |
Kubernetes describes a CronJob as a resource that “starts one-time Jobs on a repeating schedule.” The documentation identifies recurring actions such as backups and report generation as use cases. It says CronJob has been stable since Kubernetes v1.21 and its time-zone feature since v1.27; check the current Kubernetes documentation for version-specific details.
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GitHub Actions is useful for scheduling workflow runs in GitHub, but it is not a substitute for a continuously available application scheduler for every workload. Its schedule uses the default branch’s latest commit, so the code version that runs is part of the operational behavior, not just the cron expression.
How can I tell whether the scheduled work succeeded?
A schedule firing is only the first event to verify. For production work, make the outcome observable to the people responsible for it. A practical check is to ask:
- Did the scheduler create or launch a run?
- Did the process start?
- Did it finish with a successful status and produce the expected result?
- Would someone know if it failed, ran late, or never started?
Logs and scheduler status can answer some of these questions, but missed-run detection may require an explicit monitoring and alerting approach. Pick one suited to the workload and route alerts to someone able to act on them; do not assume that a scheduler’s run history is itself an alert.
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