Cron Expression Parser

What is a Cron Expression?

A cron expression is a 5-field string that defines when a scheduled task should execute. Each field represents a time component: minute, hour, day of month, month, and day of week. The name comes from the Greek word "chronos" (time), and the cron daemon has been a core Unix utility since the 1970s.

The standard 5-field format (used in Unix crontab) differs from extended formats—some systems like Quartz Scheduler use 6 or 7 fields that add seconds and/or year. This parser validates the standard 5-field Unix format.

Example breakdown: 30 14 * * 1-5 breaks down as: minute 30, hour 14 (2 PM), any day of month, any month, Monday through Friday. Result: runs at 2:30 PM every weekday.

How to Read a Cron Expression Step-by-Step

Read cron expressions left-to-right, from smallest to largest time unit:

1. Field 1 (Minute): Which minute of the hour (0-59). */15 = every 15 minutes; 0 = on the hour.
2. Field 2 (Hour): Which hour of the day in 24-hour format (0-23). 9 = 9 AM; 21 = 9 PM.
3. Field 3 (Day of Month): Which day of the month (1-31). 1 = first day; 15 = mid-month.
4. Field 4 (Month): Which month (1-12 or JAN-DEC). 1 or JAN = January.
5. Field 5 (Day of Week): Which weekday (0-7 where 0 and 7 = Sunday, or SUN-SAT). 1-5 = weekdays only.

When both day-of-month and day-of-week are specified (not *), most cron implementations run on either match—this catches many developers off guard.

Cron Expressions by Platform

• Linux/Unix crontab: Standard 5-field format. Edit with crontab -e. Jobs run in minimal shell environment—always use absolute paths.
• AWS EventBridge/CloudWatch: Uses 6-field format with year. Schedules are evaluated in UTC by default.
• GitHub Actions: Standard 5-field format in schedule triggers. All times are UTC—no timezone override.
• Kubernetes CronJobs: Standard 5-field format. Timezone support added in v1.24+ via spec.timeZone.
• Quartz Scheduler (Java): 6-7 field format with seconds and optional year. Supports L (last), W (weekday), and # (nth weekday) modifiers.

Common Cron Expression Mistakes

• Forgetting timezone: Cron runs in the server's local timezone. A job set to 0 9 * * * fires at 9 AM server time, not your local time. Always verify your server's timezone with timedatectl or date.
• DST gaps and overlaps: During daylight saving transitions, jobs scheduled between 2-3 AM may skip or run twice. Schedule critical jobs outside this window or use UTC.
• Day-of-month + day-of-week conflict: 0 9 15 * 1 runs on the 15th and every Monday—not "the 15th if it's a Monday." Use ? in systems that support it to disable one field.
• Paths and environment: Cron jobs run with minimal PATH. Scripts that work in terminal often fail in cron. Use absolute paths: /usr/bin/python3 /home/user/script.py.
• Email flooding: Cron emails output to the user by default. Add >/dev/null 2>&1 to suppress output, or redirect to a log file for debugging.