Cron expressions in practice

What a cron expression is

A cron expression is a compact schedule: fields for minute, hour, day of month, month, and day of week (some systems add a seconds field at the front). Special characters such as * (any), , (list), - (range), and / (step) combine to mean “every N units” or “on these days.” The expression does not run anything by itself — a cron daemon, CI scheduler, or cloud job runner interprets it.

Field order (common five-field form)

┌──────── minute (0–59)
│ ┌────── hour (0–23)
│ │ ┌──── day of month (1–31)
│ │ │ ┌── month (1–12)
│ │ │ │ ┌ day of week (0–7, Sunday often 0 or 7)
│ │ │ │ │
* * * * *

Always confirm whether your platform uses five fields, six (with seconds), or a proprietary variant (AWS, Kubernetes CronJob, GitHub Actions all have nuances). Fieldkit’s generator targets the familiar Unix-style patterns and explains them in plain language — open the Cron Generator to compose or decode interactively.

Practical examples

• 0 9 * * 1-5 — 09:00 every weekday (Monday–Friday).
• */15 * * * * — every 15 minutes.
• 0 0 1 * * — midnight on the first day of each month.
• 30 2 * * 0 — 02:30 every Sunday (confirm Sunday numbering on your system).
• 0 */6 * * * — on the hour, every six hours.

When day-of-month and day-of-week are both restricted (not *), behavior differs by implementation: some run when either matches, some require both. Read your runner’s docs before combining them.

Common pitfalls

• Time zones. Servers often run in UTC. “9am” in the expression may not be 9am where your users live unless the runner is configured for that zone.
• Sunday numbering. Some systems use 0–6, others allow 7 for Sunday, and names like SUN appear only in certain dialects.
• February 30. Day-of-month values that never occur in a month simply do not fire — easy to miss in quarterly jobs.
• Steps vs lists. */10 in minutes is every ten minutes from 0; that is not always the same as an arbitrary list you had in mind.
• Seconds field surprise. Pasting a six-field expression into a five-field crontab shifts meaning. Count the fields before you save.

Debugging a schedule

Write the intended plain-English rule first (“weekdays at 09:00 America/Sao_Paulo”), then translate to fields, then verify on a calendar for the next two weeks — including a month boundary. Use Fieldkit to decode an unfamiliar line from a teammate’s crontab before you change production.

For related time conversions (Unix epoch ↔ human dates), see the Timestamp Converter.

Open the live tool

Open Cron Generator — build or decode an expression in your browser. Related: FAQ · JSON pretty-print guide.