Cron expression

A cron expression is a compact schedule of five space-separated fields (minute, hour, day of month, month and day of week) that tells the cron daemon, and many schedulers modeled on it, when to run a job. Different systems add fields or change the rules, so the same text can mean different things.

The five fields

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

The cron daemon wakes every minute and runs each job whose fields all match the current time. 30 9 * * 1-5 means minute 30, hour 9, any day of the month, any month, Monday through Friday: 09:30 on weekdays.

Each field accepts a few building blocks:

Syntax	Meaning	Example
`*`	Every value	`* * * * *` runs every minute
`a,b`	A list	`0,30` is minutes 0 and 30
`a-b`	An inclusive range	`9-17` is hours 9 to 17
`*/n` or `a-b/n`	A step	`*/15` is every 15 minutes

Most implementations also accept @daily, @hourly, @weekly, @monthly and @yearly as shortcuts.

Two rules that surprise people

Day of month and day of week combine with OR. When both fields are restricted, cron runs a job when either matches. 0 0 13 * 5 runs on the 13th of every month and on every Friday, not just on Friday the 13th. It is an AND only when one of the two fields is *.
Steps restart each cycle. */7 * * * * fires at minutes 0, 7, 14 … 56 and then at 0 again, so the gap after minute 56 is only 4 minutes. A step never carries over from one hour to the next.

Dialects

Standard five-field cron is used by crontab, Kubernetes CronJobs and GitHub Actions schedules. Other systems borrow the idea but change it. Quartz and Spring put a seconds field first and use ?, L, W and #. AWS EventBridge uses six fields ending in a year. Both number the days of the week from Sunday = 1 instead of 0. An expression copied between these systems can run at the wrong time without any error.

Common pitfalls

Time zones. Cron uses the time zone of the machine running it, and containers and cloud schedulers are commonly UTC.
Percent signs. In a crontab command, an unescaped % is treated as a newline, so write \%.
No catch-up. If the machine is off at the scheduled minute, plain cron skips the run.
Minimal environment. Jobs run with a short PATH and no shell profile, so commands that work in your terminal can fail under cron.

Unix timestamp — A Unix timestamp is the number of seconds that have passed since 00:00:00 UTC on 1 January 1970, the Unix epoch. It is a single number that identifies a moment in time regardless of time zone, which makes it easy to store, sort and compare.
ISO 8601 timestamp — ISO 8601 is the international standard for writing dates and times unambiguously, and RFC 3339 is the strict internet profile of it, such as 2026-09-18T14:30:00Z. Both put the largest unit first, so the text sorts in chronological order.