Cron Expression Explainer: Frequently Asked Questions

Standard Unix cron uses 5 fields: minute, hour, day-of-month, month, day-of-week. Many modern frameworks (Quartz, Spring, AWS EventBridge, GitHub Actions) add a sixth field at the beginning for seconds. Always check your platform's documentation.

The / character means 'step'. */15 in the minutes field means 'every 15 minutes' (0, 15, 30, 45). */2 in the hours field means 'every 2 hours' (0, 2, 4, 6... 22). 1/15 would mean 'every 15 starting from 1' (1, 16, 31, 46).

Standard 5-field cron has minute-level precision. To run every 30 seconds, you need either a 6-field cron implementation that includes a seconds field, or two separate cron jobs: one at */1 * * * * and one at 30 * * * * (offset by 30 seconds).

Cron has approximately 1-minute precision for standard implementations. Jobs may start up to 60 seconds after the scheduled minute due to the cron daemon's check interval. For second-level precision, use a dedicated job scheduler rather than cron.

These are cron shorthand strings: @daily = 0 0 * * * (midnight every day), @weekly = 0 0 * * 0 (midnight every Sunday), @monthly = 0 0 1 * * (midnight on the 1st of each month), @yearly/@annually = 0 0 1 1 * (midnight January 1st), @hourly = 0 * * * * (every hour at :00), @reboot = run once at system startup.

Month names (JAN–DEC) and day names (SUN–SAT) can be used instead of numbers in some implementations and are case-insensitive. Numbers are always accepted. The explainer accepts both numeric and named values.