What is cron?
Cron is a time-based job scheduler in Unix-like operating systems. The name comes from the Greek word for time, chronos. It runs background tasks (called cron jobs) at specified intervals — anything from "every minute" to "on the first Monday of every quarter at 3 AM". Cron syntax is also used in GitHub Actions, AWS CloudWatch Events, Azure Logic Apps, Kubernetes CronJobs, and many application-level schedulers like Quartz (.NET/Java) and node-cron.
The 5-field cron format
A standard cron expression has five fields separated by spaces:
┌──────────── minute (0–59)
│ ┌─────────── hour (0–23)
│ │ ┌──────── day of month (1–31)
│ │ │ ┌───── month (1–12)
│ │ │ │ ┌── day of week (0–7, 0 and 7 both mean Sunday)
│ │ │ │ │
* * * * *Special characters
*(asterisk) — matches every possible value in that field.*in the hour field means "every hour".,(comma) — lists multiple values.1,3,5in the day-of-week field means Monday, Wednesday, Friday.-(hyphen) — defines a range.9-17in the hour field means hours 9 through 17 inclusive./(slash) — specifies a step.*/5in the minute field means "every 5 minutes".0/15means "at 0, 15, 30, 45".
Practical examples
| Expression | Meaning |
|---|---|
| * * * * * | Every minute |
| 0 * * * * | Every hour at minute 0 |
| 0 0 * * * | Every day at midnight |
| 0 9 * * 1-5 | Weekdays at 9:00 AM |
| */15 * * * * | Every 15 minutes |
| 0 0 1 * * | First day of every month at midnight |
| 0 0 * * 0 | Every Sunday at midnight |
| 30 8 * * 1 | Every Monday at 8:30 AM |
| 0 0 1 1 * | Once a year, 1 Jan at midnight |
| 0 9,17 * * 1-5 | Weekdays at 9 AM and 5 PM |
The 6-field format (with seconds)
Some schedulers — notably Quartz (.NET and Java), Spring Scheduler, and some cloud platforms — add a seconds field at the beginning, making it a 6-field expression:
┌──────────── second (0–59)
│ ┌─────────── minute (0–59)
│ │ ┌──────── hour (0–23)
│ │ │ ┌───── day of month
│ │ │ │ ┌── month
│ │ │ │ │ ┌─ day of week
│ │ │ │ │ │
0 * * * * *This is a common source of confusion: the same string 0 * * * * means "every hour at minute 0" in 5-field cron, but means "at second 0 of every minute, every hour" in 6-field Quartz format. Always check which format your scheduler expects.
Common pitfalls
- Timezone confusion — cron runs in the server's local timezone by default. If your server is in UTC and your users are in IST (UTC+5:30), a job scheduled at
0 2 * * *runs at 7:30 AM IST, not 2:00 AM. Use a timezone-aware scheduler or specify the timezone explicitly. - Day-of-month AND day-of-week interaction — in most cron implementations, if both day-of-month and day-of-week are specified (not
*), the job runs when either condition is met, not when both are. This surprises many developers. - Minute-level granularity — standard 5-field cron cannot schedule tasks more frequently than once per minute. For sub-minute jobs, use your language's task runner or a dedicated scheduler.
- Missed runs — if the cron daemon was not running when a job was due (server was down), most implementations simply skip that run. Use a scheduler with catch-up logic if missed runs must execute.