website/content/en/docs/concepts/workloads/controllers/cron-jobs.md

---
reviewers:
- erictune
- soltysh
- janetkuo
title: CronJob
content_template: templates/concept
weight: 80
---

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A _Cron Job_ creates [Jobs](/docs/concepts/workloads/controllers/jobs-run-to-completion/) on a time-based schedule.

One CronJob object is like one line of a _crontab_ (cron table) file. It runs a job periodically
on a given schedule, written in [Cron](https://en.wikipedia.org/wiki/Cron) format.

{{< note >}}
All **CronJob** `schedule:` times are based on the timezone of the master where the job is initiated.
{{< /note >}}

For instructions on creating and working with cron jobs, and for an example of a spec file for a cron job, see [Running automated tasks with cron jobs](/docs/tasks/job/automated-tasks-with-cron-jobs).

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## Cron Job Limitations

A cron job creates a job object _about_ once per execution time of its schedule. We say "about" because there
are certain circumstances where two jobs might be created, or no job might be created. We attempt to make these rare,
but do not completely prevent them. Therefore, jobs should be _idempotent_.

If `startingDeadlineSeconds` is set to a large value or left unset (the default)
and if `concurrencyPolicy` is set to `Allow`, the jobs will always run
at least once.

For every CronJob, the CronJob controller checks how many schedules it missed in the duration from its last scheduled time until now. If there are more than 100 missed schedules, then it does not start the job and logs the error

````
Cannot determine if job needs to be started. Too many missed start time (> 100). Set or decrease .spec.startingDeadlineSeconds or check clock skew.
````

It is important to note that if the `startingDeadlineSeconds` field is set (not `nil`), the controller counts how many missed jobs occurred from the value of `startingDeadlineSeconds` until now rather than from the last scheduled time until now. For example, if `startingDeadlineSeconds` is `200`, the controller counts how many missed jobs occurred in the last 200 seconds.

A CronJob is counted as missed if it has failed to be created at its scheduled time. For example, If `concurrencyPolicy` is set to `Forbid` and a CronJob was attempted to be scheduled when there was a previous schedule still running, then it would count as missed.

For example, suppose a CronJob is set to schedule a new Job every one minute beginning at `08:30:00`, and its
`startingDeadlineSeconds` field is not set. If the CronJob controller happens to
be down from `08:29:00` to `10:21:00`, the job will not start as the number of missed jobs which missed their schedule is greater than 100.

To illustrate this concept further, suppose a CronJob is set to schedule a new Job every one minute beginning at `08:30:00`, and its
`startingDeadlineSeconds` is set to 200 seconds. If the CronJob controller happens to
be down for the same period as the previous example (`08:29:00` to `10:21:00`,) the Job will still start at 10:22:00. This happens as the controller now checks how many missed schedules happened in the last 200 seconds (ie, 3 missed schedules), rather than from the last scheduled time until now.

The CronJob is only responsible for creating Jobs that match its schedule, and
the Job in turn is responsible for the management of the Pods it represents.

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