Cron Expression Explainer & Parser

What is a cron expression?

Cron is the Unix/Linux standard for scheduled tasks — running commands automatically at fixed times. A cron expression has 5 fields:

┌─── minute (0-59)
│ ┌─── hour (0-23)
│ │ ┌─── day of month (1-31)
│ │ │ ┌─── month (1-12)
│ │ │ │ ┌─── day of week (0-7, 0 and 7 = Sunday)
│ │ │ │ │
* * * * *  command

Special characters

• * — any value (every).
• , — list. E.g. 1,3,5 = at 1, 3, 5.
• - — range. E.g. 1-5 = from 1 to 5.
• / — step. E.g. */15 = every 15 units.

When to use cron

• Linux server: crontab -e — nightly backups, log rotation.
• GitHub Actions: schedule: - cron: '0 9 * * 1-5' — run CI at 9 AM Mon-Fri.
• AWS EventBridge / Lambda: trigger functions on a schedule.
• Vercel Cron: invoke serverless functions periodically.
• Database jobs: PostgreSQL pg_cron, MySQL events.

Common pitfalls

• Timezone: Linux cron uses the server's clock (often UTC). Vercel/AWS also UTC. Adjust if you want a specific local time.
• Day-of-month vs Day-of-week: when BOTH are not *, POSIX defines it as OR (runs if either matches). The tool handles this correctly.
• Sunday = 0 or 7: both are valid in cron.
• Step start point: */15 starts from 0 → 0,15,30,45 (not 15,30,45,60).