Cron Expressions — A Practical Guide with Examples & Next Run Times

# Cron Jobs: How to Read Those Star and Slash Patterns

Cron is a Linux scheduler that runs tasks at fixed times. If you've ever seen 0 9 * * 1-5 and wondered what it means, this is for you.

The 5 Fields

minute  hour  day-of-month  month  day-of-week

Patterns I Use Regularly

Every weekday at 9am: ` 0 9 * * 1-5 `

Every 15 minutes: ` */15 * * * * `

First of every month at midnight: ` 0 0 1 * * `

Every Sunday at 2am: ` 0 2 * * 0 `

Every 6 hours: ` 0 */6 * * * `

The Tricky Parts

Day of month vs day of week: If both are specified (not *), cron runs when EITHER matches. 0 0 13 * 5 runs on the 13th AND on Fridays, not just Friday the 13th.

*/N doesn't always mean every N units: */10 in the minute field means minutes 0, 10, 20, 30, 40, 50. But if you start at 3, there's no 3-59/10 shortcut in standard cron.

Timezones matter: Cron runs in the server's timezone. If your server is in UTC and you want 9am EST, that's 0 14 * * * (14:00 UTC = 9:00 EST).

Parsing Cron Expressions

Reading cron takes practice. If you're debugging a schedule or want to confirm what a expression does, use our Cron Parser — paste the expression, get a human-readable description and the next 5 execution times. Free, no signup.

## Cron Expression Fields Explained

A cron expression has five fields (in traditional cron) or six (in some modern systems with seconds):

# Traditional 5-field cron
# minute hour day-of-month month day-of-week
0 9 * * 1-5        # 9:00 AM, Monday through Friday
0 */2 * * *        # Every 2 hours
0 0 1 * *          # Midnight on the 1st of every month

Common Cron Patterns

• 0 0 * * * — Daily at midnight
• 0 0 * * 0 — Every Sunday at midnight (weekly)
• 0 0 1 * * — First day of every month at midnight
• 0 9 * * 1-5 — Weekdays at 9:00 AM
• */15 * * * * — Every 15 minutes
• 0 0 1 1 * — January 1st at midnight (yearly)
• 0 0 L * * — Last day of every month (L = last)

Cron Pitfalls

1. Day of month vs day of week: If both are specified (not *), cron runs when EITHER matches, not both. 0 0 13 * 5 runs on Friday the 13th AND every Friday, not just Friday the 13th.
2. Timezone issues: Cron uses the server timezone. If your server is in UTC but your users are in New York, 9:00 AM cron runs at 5:00 AM EST.
3. DST transitions: When daylight saving time changes, cron jobs may run twice or not at all. Use UTC to avoid this.
4. Long-running jobs: If a job takes longer than the interval, multiple instances may overlap. Use a lock file or a job queue.