Cron Expression Validator

A standard cron expression has 5 fields separated by spaces: minute (0–59), hour (0–23), day of month (1–31), month (1–12), day of week (0–6, where 0 is Sunday). For example, '0 9 * * 1-5' means 'at 9:00 AM, every weekday (Monday through Friday)'.

* means 'every value' (wildcard). / is a step value — */5 in the minute field means every 5 minutes. - defines a range — 1-5 in day-of-week means Monday through Friday. , is a list separator — 1,15 in the day-of-month field means the 1st and 15th of every month.

If you specify a value in both day-of-month and day-of-week (not using *), most cron implementations treat them as a union — the job runs if either condition matches. If only one is specified (the other is *), only that condition is used. Use ? in one field when you intend to restrict by only the other.

Use */15 * * * * — the */15 step in the minute field means the job runs at minute 0, 15, 30, and 45 of every hour. This pattern works in standard cron, GitHub Actions (on.schedule), Kubernetes CronJobs, and AWS EventBridge.

Standard 5-field cron is POSIX-compatible. Some tools like Quartz Scheduler (Java), AWS CloudWatch Events, and some node schedulers add a 6th field for seconds (0–59) at the start, or a 7th field for year. This validator uses the standard 5-field format compatible with Unix cron, crontab, GitHub Actions, and Kubernetes.

The most common causes are timezone mismatches (cron runs in the server's local timezone, not yours), off-by-one in day-of-week (0=Sunday vs 1=Sunday depending on the system), and DST transitions that skip or repeat an hour. Always confirm the timezone of your cron daemon and consider using UTC for reliability.