What is a Cron Job?

How cron works, how to read and write cron expressions, and common scheduling patterns.

What is a cron job?

A cron job is a scheduled task that runs automatically at a specified time or interval on Unix-based systems. The name comes from Chronos (the Greek god of time) and the daemon that runs the jobs is called cron.

Cron jobs are used for: database backups, sending scheduled emails, generating reports, cleaning up old files, refreshing caches, running health checks, and any task that needs to happen on a schedule without manual intervention.

Cron expression syntax

A 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   (0–6, Sunday=0)
│ │ │ │ │
* * * * *

Special characters: * (any value) · , (list) · - (range) · / (step)

Common cron expressions

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
0 0 * * 0	Every Sunday at midnight
0 0 1 * *	First day of every month at midnight
/15 * * *	Every 15 minutes
0 9,17 * * *	At 9 AM and 5 PM every day
0 0 1 1 *	January 1st at midnight (yearly)
30 2 * * 6	Every Saturday at 2:30 AM

Special shortcuts

Many cron implementations support aliases:

@yearly   → 0 0 1 1 *     (once a year)
@monthly  → 0 0 1 * *     (once a month)
@weekly   → 0 0 * * 0     (once a week, Sunday)
@daily    → 0 0 * * *     (once a day, midnight)
@hourly   → 0 * * * *     (once an hour)
@reboot   → run once on startup

Cron Expression Parser

Paste any cron expression to see a plain-English explanation and next run times.

Open tool →

Frequently asked questions

What timezone does cron use?: By default, cron uses the system timezone of the server. This can cause confusion when servers are in UTC but you're thinking in local time. Always specify your intent clearly — "9 AM UTC" rather than "9 AM" — and use a cron parser to verify the next scheduled run.
What happens if a cron job is still running when the next one starts?: By default, cron doesn't track whether a previous run finished — it will start a new instance regardless. This can cause overlapping runs. Use a lock file or a job queue system if you need to prevent concurrent execution.
Where are cron jobs defined?: On Linux/Mac, user-level cron jobs are defined in a crontab file (edit with crontab -e). System-level jobs go in /etc/cron.d/, /etc/cron.daily/, etc. Modern cloud platforms (Vercel, AWS Lambda, Railway) have their own cron scheduling that doesn't require a crontab.