Cron Expression Generator

Reading the five fields

┌───────────── minute        (0 – 59)
│ ┌─────────── hour          (0 – 23)
│ │ ┌───────── day of month  (1 – 31)
│ │ │ ┌─────── month         (1 – 12 or JAN – DEC)
│ │ │ │ ┌───── day of week   (0 – 6, Sunday = 0, or SUN – SAT)
│ │ │ │ │
30 3 * * 1-5     → 03:30, Monday to Friday

Step values start at the field boundary

*/20 * * * * runs at :00, :20 and :40 because the step starts at zero in the minute field. It does not start when the job is installed. Likewise, */45 runs at :00 and :45, followed by :00 in the next hour, so one interval is only 15 minutes.

For an even interval that does not divide the hour, run more frequently and check elapsed time inside the job, or use an interval-based scheduler such as a systemd timer.

Which timezone does cron use?

A system crontab uses the server’s configured timezone, which is often UTC on cloud instances. Confirm that timezone before translating a local business time into a cron expression.

• Set the timezone explicitly. Vixie cron supports CRON_TZ=Europe/London at the top of a crontab, while some other implementations use TZ=.
• Kubernetes CronJobs: Kubernetes v1.27 and later support .spec.timeZone. Without it, scheduling follows the controller manager’s timezone.
• Account for daylight-saving changes. Depending on the cron implementation, a time in the transition window may be skipped or repeated. Use UTC or choose a time outside the local transition window for jobs that must run once.

Six-field scheduler formats

Quartz and Spring’s @Scheduled syntax use six fields by adding seconds at the front. AWS EventBridge also uses six fields, but its final field is a year, and one of its day fields uses ? instead of *.

Standard (5)      30 3 * * 1-5          03:30 on weekdays
Quartz (6)      0 30 3 ? * MON-FRI      leading seconds, ? for unused day field
EventBridge     30 3 ? * MON-FRI *      trailing year

A valid expression for one scheduler may be invalid or mean something different in another. This tool validates the standard five-field format only.

Making cron failures visible

# Open the current user crontab
crontab -e

    # Display the current crontab
crontab -l

    # Send output to a log because cron mail is often unread
    # This keeps failures visible
30 3 * * 1-5 /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

    # Escape percent signs in crontab commands
0 4 * * * /usr/bin/pg_dump db > /backups/db-$(date +\%Y\%m\%d).sql

• Use absolute paths. Cron often has a minimal PATH, such as /usr/bin:/bin. Use full paths for commands that may not be available there.
• Load required environment explicitly. Do not assume cron reads .bashrc, .profile, nvm configuration or a virtual environment.
• Escape %. A percent sign represents a newline in a crontab. An unescaped date +%Y can truncate the command.
• End the file with a newline. Some cron implementations ignore a final crontab line that has no newline.
• Prevent overlapping runs. If a job can exceed its interval, a lock such as flock -n /tmp/job.lock can stop another copy from starting.