How Cron Expressions Work

Anatomy of a cron expression

A cron expression is a single line of text with five space-separated fields. Each field controls one unit of time:

* * * * *
│ │ │ │ │
│ │ │ │ └── day of week (0–6, Sun=0)
│ │ │ └──── month (1–12)
│ │ └────── day of month (1–31)
│ └──────── hour (0–23)
└────────── minute (0–59)

A literal asterisk * means “every valid value for this field.” So * * * * * means “every minute of every hour of every day of every month, every day of the week” — the most frequent schedule possible in standard cron.

The five fields

Day-of-week 0 is Sunday in Unix and Kubernetes. Quartz and EventBridge use 1 for Sunday and 7 for Saturday. Spring accepts both 0 and 7 as Sunday.

Special characters

Beyond literal numbers and *, four characters appear in every cron dialect:

• Comma , — list multiple values. 0,15,30,45 in the minute field means “at :00, :15, :30, and :45.”
• Hyphen - — range of values. 1-5 in the day-of-week field means “Monday through Friday.”
• Slash / — step value. */15 in the minute field means “every 15th minute starting from 0” (i.e., :00, :15, :30, :45). 2/10 starting from 2 means :02, :12, :22, :32, :42, :52.
• Asterisk * — every value.

Quartz, AWS EventBridge, and a handful of other dialects add more characters: ? (no specific value), L (last day of month/week), W (weekday nearest a given day), and # (the nth occurrence of a weekday in a month). See the Cron Dialect Comparison for the full list.

Common patterns

Gotchas worth knowing

The day-of-month vs day-of-week OR trap. If both fields are set to anything other than *, the job runs on days that match either field — not both. So 0 0 1 * 1 runs at midnight on the 1st of every month and every Monday. To run “on the 1st only if it’s a Monday,” you have to gate it inside your script. See the DOM/DOW trap guide for details.

Daylight saving time. Traditional Unix cron uses the system’s local timezone, which causes a daily job scheduled at 2:30 AM to skip on spring-forward day and run twice on fall-back day. Always set CRON_TZ=UTC at the top of your crontab if your workload spans timezones. Kubernetes CronJob defaults to UTC unless spec.timeZone is set.

Minimum granularity is one minute. Cron cannot fire more often than once per minute. For sub-minute scheduling, use a long-running process with an in-process scheduler (e.g. setInterval, time.Tick, asyncio.sleep), a job queue (Celery, Sidekiq), or systemd timers with OnUnitActiveSec=30s.

Overlapping runs. Most cron daemons happily spawn a new instance while the previous one is still running. If your job can take longer than the interval, guard with a lockfile (flock), set concurrencyPolicy: Forbid on Kubernetes CronJobs, or pick a longer interval.