Cron expression for twice a day

The cron expression 0 6,18 * * * runs a job twice daily — at 6 AM and 6 PM. The comma in the second field lists two distinct hours; any field accepts a comma-separated list of values. Useful for jobs that should run in the morning and evening but not all day. AWS EventBridge uses cron(0 6,18 * * ? *).

Quick reference

How does the comma syntax work?

A cron field accepts three structural forms: a single value (5), a range (1-5), or a list (1,3,5). The comma form lists discrete values — useful when the schedule isn’t a simple step.

You can combine all three in one field. 0,30 9-17 * * 1-5 reads as “minute 0 OR minute 30, between hour 9 and hour 17, every day-of-month, every month, day-of-week 1 through 5” — every 30 minutes during business hours on weekdays. Each field is independent.

The comma list has no implicit ordering and no de-duplication. 0 6,6,18 * * * is equivalent to 0 6,18 * * * — duplicates are ignored. The parser also doesn’t validate logical sense — 0 25,30 * * * (with hour 25) is a parse error, not a silent skip.

Variations

How do I use it on each platform?

Linux crontab:

0 6,18 * * * /usr/local/bin/sync-feeds

Kubernetes CronJob:

apiVersion: batch/v1
kind: CronJob
metadata:
  name: sync-feeds
spec:
  schedule: "0 6,18 * * *"
  timeZone: "America/Los_Angeles"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
            - name: sync
              image: my-org/sync-feeds:1.0
          restartPolicy: OnFailure

AWS EventBridge Scheduler:

Type: AWS::Scheduler::Schedule
Properties:
  Name: sync-feeds
  ScheduleExpression: 'cron(0 6,18 * * ? *)'
  ScheduleExpressionTimezone: 'America/Los_Angeles'
  FlexibleTimeWindow:
    Mode: 'OFF'
  Target:
    Arn: !GetAtt SyncFunction.Arn
    RoleArn: !GetAtt SchedulerRole.Arn

GitHub Actions:

on:
  schedule:
    - cron: '0 6,18 * * *'

GitHub Actions allows multiple cron entries under schedule — equivalent to a comma list, but more readable when the times are unrelated:

on:
  schedule:
    - cron: '0 6 * * *'      # morning sync
    - cron: '0 18 * * *'     # evening sync

Common mistakes

Whitespace inside the comma list. 0 6, 18 * * * (note the space after the comma) is rejected by most cron parsers because they tokenize on whitespace. Comma lists must be contiguous: 0 6,18 * * *.

Confusing 6,18 with 6-18. 6,18 is exactly two values (6 and 18). 6-18 is a range — every hour from 6 through 18 inclusive (13 values). Easy to mistype when you mean “twice a day at the boundaries”.

Using 0 6,18 * * * and expecting “exactly 12 hours apart”. It fires at 6:00 and 18:00 — 12 hours apart. But 0 9,17 * * * (9 AM and 5 PM) is 8 hours apart in the morning gap and 16 hours in the evening gap. Cron lists discrete clock times; it doesn’t compute spacing for you.

For other schedules see common cron schedules, or build a custom expression with the Cron Expression Builder.