Korean Cron Expression Translator

Cron expression syntax guide

What cron is, how the five fields combine, and the real-world schedules people actually use.

1. What is cron?

cron is the Unix time-based scheduler and, by extension, the notation used to describe a schedule. Brian Kernighan designed it in 1975, and today Linux, macOS, BSD, Kubernetes CronJobs, GitHub Actions schedule:, AWS EventBridge Cron, Jenkins and countless other systems all reuse the same field syntax.

2. POSIX five fields

┌──────── minute        0-59
│ ┌────── hour          0-23
│ │ ┌──── day of month  1-31
│ │ │ ┌── month          1-12   or JAN-DEC
│ │ │ │ ┌ day of week    0-6    (0 or 7 = Sunday) or SUN-SAT
* * * * *

For example 0 9 * * 1-5 means minute=0, hour=9, any day of month, any month, Monday through Friday — so "weekdays at 9 AM on the dot".

3. Five operators

  • * — every value in the field.
  • a,b,c — list.
  • a-b — range.
  • */n — step. Combine with a range: 0-30/5 * * * *.
  • JAN..DEC, SUN..SAT — three-letter aliases.

4. Shortcut keywords

KeywordEquivalent
@yearly0 0 1 1 *
@monthly0 0 1 * *
@weekly0 0 * * 0
@daily0 0 * * *
@hourly0 * * * *

5. Day-of-month and day-of-week both set: the trap

When both fields are restricted, Vixie/Linux cron treats them as OR (fire if either matches). Quartz (Java) treats them as AND. POSIX sides with OR. This tool follows POSIX. If you want a single condition, leave the other field as*.

6. Ten real-world schedules

  • 0 9 * * 1-5 — weekdays at 9 AM.
  • */15 * * * * — every 15 minutes.
  • 0 0 * * 0 — weekly Sunday midnight.
  • 0 2 * * * — daily at 2 AM (traditional DB backup slot).
  • 0 0 1 * * — first of every month at midnight.
  • 30 18 * * 5 — Friday 6:30 PM retro reminder.
  • 0 */6 * * * — every 6 hours.
  • 0 9,18 * * * — 9 AM and 6 PM daily.
  • 0 0 1 1 * — New Year midnight.
  • Split weekday 9 AM + weekend 10 AM into two separate entries.

7. Runtime quirks by platform

  • Linux cron — uses the system timezone. Put CRON_TZ=Asia/Seoul at the top of the crontab if the host is UTC.
  • Kubernetes CronJob — defaults to UTC. Set spec.timeZone (Kubernetes 1.27+).
  • GitHub Actions — UTC only; there is no timezone override. Offset the expression instead.
  • AWS EventBridge — six-field dialect with ?, L, W,#. Not drop-in compatible with POSIX.
  • Quartz — six or seven fields (seconds and optionally year), cannot specify day and weekday simultaneously.

8. Debugging checklist

  1. Paste the expression here and read the plain-language description first.
  2. Scan the next-10-runs table and make sure the cadence matches intent.
  3. Confirm whether production runs in KST or UTC — the 9-hour offset mistake is the most common one.
  4. Remember Vixie vs Quartz day/weekday OR-vs-AND semantics.
  5. If logs show nothing fired, check the cron daemon status (systemctl status cron).

9. Scope of this tool

This release covers the POSIX 5-field syntax. Quartz seconds, L, W, #, AWS EventBridge 6-field and a dedicated GitHub Actions tab are on the roadmap.

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