Cron Expressions Explained

The five fields

┌───────────── minute        (0–59)
│ ┌─────────── hour          (0–23)
│ │ ┌───────── day of month  (1–31)
│ │ │ ┌─────── month         (1–12 or JAN–DEC)
│ │ │ │ ┌───── day of week   (0–6, Sunday = 0, or SUN–SAT)
│ │ │ │ │
* * * * *   → every minute
30 2 * * *  → at 02:30 every day
0 9 * * 1-5 → at 09:00 Monday to Friday

Try it: Cron Expression Explainer

The operators: *, /, - and ,

• * (asterisk) — every value. * in the hour field means every hour.
• , (comma) — a list. 0,30 in the minute field means at minute 0 and minute 30.
• - (hyphen) — a range. 9-17 in the hour field means 9, 10, … 17.
• / (slash) — a step. */15 in the minute field means every 15 minutes (0, 15, 30, 45). In the day-of-month field, 1-31/2 (or */2) means the odd-numbered days — 1, 3, 5 … 31 — so a job that runs on the 31st runs again on the 1st: not a strict every-other-day interval.
• Names — JAN–DEC and SUN–SAT are accepted in most implementations and are easier to read than numbers.
• Extensions — some schedulers accept L (last), W (nearest weekday), # (nth weekday) and ? — these are Quartz/Spring features, not standard cron.

Try it: Cron Expression Generator

Common schedules

*/5 * * * *      every 5 minutes
0 * * * *        every hour, on the hour
0 */6 * * *      every 6 hours (00:00, 06:00, 12:00, 18:00)
0 0 * * *        every day at midnight
0 9 * * 1-5      weekdays at 09:00
0 9 * * 1        every Monday at 09:00
0 0 1 * *        first day of every month at midnight
0 0 1 1 *        once a year, 1 January
30 23 * * 0      Sundays at 23:30
0 8,20 * * *     twice a day, 08:00 and 20:00
15 3 */2 * *     odd-numbered days of the month at 03:15
0 0 * * 5        every Friday at midnight

Try it: Cron Expression Generator Try it: Cron Expression Validator

Time zones: the most common cron bug

A cron expression has no time zone of its own; it runs in whatever zone the scheduler uses. Linux crontab uses the system zone; Cloudflare Workers use UTC; GitHub Actions uses UTC unless the schedule sets a timezone; a Kubernetes CronJob uses the kube-controller-manager's local zone (often UTC on managed clusters) unless it sets .spec.timeZone; and cloud services vary. A job meant for 09:00 Riyadh time (UTC+3) must be written as 0 6 * * * on a UTC scheduler — and 09:00 in a zone that observes daylight saving shifts by an hour twice a year. Always document the zone next to the expression and convert explicitly.

Try it: Timezone Converter

Day of month vs day of week

When both day fields are restricted, standard cron runs the job if either matches (OR), not both. 0 0 13 * 5 fires on every 13th and on every Friday, not only on Friday the 13th. To express "second Tuesday of the month" or "last day of the month" you need Quartz-style extensions or a small check inside the job itself.

Try it: Cron Expression Explainer

Six-field cron and other variants

• Quartz, Spring and some Java schedulers put a seconds field first: 0 */5 * * * ? — six or seven fields. Pasting a five-field expression there shifts every field by one.
• AWS EventBridge scheduled rules use six fields — minute, hour, day of month, month, day of week and a year field — and require ? in one of the two day fields, with day of week counted 1–7 from Sunday: cron(0 9 ? * MON-FRI *). A five-field expression is rejected there.
• Cloudflare Workers, Kubernetes, GitHub Actions and Vercel use the standard five fields.
• Some systems accept @hourly, @daily, @weekly, @monthly and @yearly as shorthand.
• Minimum resolution is one minute; sub-minute schedules need a different mechanism.

Try it: Cron Expression Validator

Testing a cron expression

Before deploying, paste the expression into a validator that lists the next several run times in your zone. Check that the first run is when you expect, that the interval between runs is right, and that month-end and weekend cases behave. Then make the job itself idempotent — safe to run twice — because every scheduler will eventually run it twice or skip it once.

Try it: Cron Expression Validator Try it: Unix Timestamp Converter

Frequently asked questions

References

• crontab — POSIX.1-2017 (IEEE Std 1003.1-2017)The Open Group · 2017The portable definition of the five-field crontab line and the meaning of each field.
• crontab(5) — Linux manual pageman7.orgThe Linux (cron/Vixie) crontab format, including ranges, steps, lists and the @hourly-style shortcuts.