Cron Expression Parser

Cron is a time-based job scheduler that executes commands or scripts at specified intervals. A cron expression is a string of five fields (sometimes six) that define the schedule: minute (0–59), hour (0–23), day of month (1–31), month (1–12), and day of week (0–7, where both 0 and 7 represent Sunday). Each field accepts specific values, ranges (1-5), lists (1,3,5), step values (*/15 for every 15 units), and wildcards (*).

Common cron expressions: 0 * * * * runs at the top of every hour. */15 * * * * runs every 15 minutes. 0 9 * * 1-5 runs at 9:00 AM on weekdays. 0 0 1 * * runs at midnight on the first of every month. 0 2 * * 0 runs at 2 AM every Sunday. The special strings @hourly, @daily, @weekly, @monthly, and @reboot are supported by many cron implementations as convenient aliases.

Cron is supported natively in Linux via crontab, and is used in Kubernetes (CronJob), GitHub Actions (on: schedule), AWS EventBridge, and most backend scheduling systems. A common mistake is specifying both day-of-month and day-of-week fields with non-wildcard values — most cron implementations treat this as OR (run if either condition is true) rather than AND. Timezone handling is another common pitfall: cron schedules run in the system timezone unless configured otherwise, which can produce unexpected behavior when servers are in UTC but schedules are designed for local time.