Twice a Day Cron – Free, Online, Private

The expression

0 8,20 * * * is a five-field cron expression: minute, hour, day of month, month, day of week. There is no seconds field and no year field. At 08:00 and 20:00, every day The next five run times on this page are computed in your browser with the same expression, in UTC, so a reload later in the hour shows a later list. Edit the box if you want to nudge a field and watch the list change, or open the line in the cron parser.

What each field is doing

The minute field is the single value 0. No other value in the 0–59 range matches. The hour field is the list 8,20. Only those 0–23 values match. A list does not fill in the numbers between the commas. The day-of-month field is an asterisk, so every value in the 1–31 range is allowed. This field is not what limits the schedule. The month field is an asterisk, so every value in the 1–12 range is allowed. This field is not what limits the schedule. The day-of-week field is an asterisk, so every value in the 0–6, Sunday as 0 or 7 range is allowed. This field is not what limits the schedule.

When the day-of-month and day-of-week fields are both something other than *, many cron implementations run the job if either field matches. This preset leaves at least one of those two fields as *, so you do not have to resolve that rule to predict it. Names such as MON and JAN are readable and widely accepted. The snippets use numbers so a strict parser does not have to know the names.

Why this schedule

A list of two hours is the honest way to say 'twice a day' when the times are not twelve hours apart from midnight. 08:00 and 20:00 are a common pair: once before a business day, once in the evening. The minute is 0 for both. You cannot set a different minute per hour in one expression.

A morning and evening sync, a twice-daily backup of a small database, and a status ping to a status page are the fits. A job that must run every twelve hours from the moment of the last failure needs a queue, not cron.

Where it goes wrong

The comma is a list of hours, not a range from 8 to 20. An hour field of 8-20 would run every hour from 08:00 through 20:00, which is thirteen runs. If you wanted a twelve-hour step aligned to midnight, use */12. The next five runs should alternate 08:00 and 20:00. If they do not, the field was edited.

A sixth field is the seconds field used by Quartz, some Spring apps, and some Kubernetes setups that are not the classic CronJob schedule. Classic crontab, GitHub Actions, and the schedule field of a batch/v1 CronJob are five fields. If you paste a six-field line into those systems, the extra token shifts every field one place to the left and the job runs on a different day. L, W, and the # 'nth weekday' modifier are real in some daemons and rejected or reinterpreted in others. They are not used on this page. The English description and the next runs both refuse to guess at a token the calculator does not share with every target.

Crontab, GitHub Actions, and Kubernetes

GitHub Actions accepts this cadence. The schedule is UTC, it can start late when the service is busy, and it is dropped if the repository has had no activity for sixty days. The workflow file only needs the expression. It does not need a seconds field.

A user crontab does not include the username. A system crontab, the kind in /etc/crontab, inserts the user between the expression and the command. The line below is the user form. Kubernetes CronJob has used spec.timeZone since 1.27. The manifest sets it to Etc/UTC so the schedule matches the next-run list. Remove the field on a cluster older than that, and know that the schedule then uses the controller's zone.

crontab0 8,20 * * * /usr/local/bin/job

GitHub Actionson:
  schedule:
    - cron: "0 8,20 * * *"

Kubernetes CronJobapiVersion: batch/v1
kind: CronJob
metadata:
  name: twice-a-day
spec:
  schedule: "0 8,20 * * *"
  timeZone: "Etc/UTC"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
            - name: job
              image: busybox:1.36
              args: ["echo", "twice-a-day"]
          restartPolicy: OnFailure

• Keep the command idempotent. Cron does not remember a failure and it does not skip a run that is already in progress.
• Write the timezone next to the expression. UTC in the file and local time in the ticket are how a job fires eight hours early.
• Offset the minute if several jobs would otherwise start at once.