Cron Expression Builder

Build a schedule in three steps

1. Start from a preset such as Daily at 09:00 or Weekdays 9 to 5, or type an expression straight into the raw box.
2. Fine-tune each field with the builder. Every field can run on every value, on specific values you tick, over a range, on a step (every N), or from a free-form list.
3. Read the plain-English description and check the next 10 run times. If they match what you meant, copy the expression or the crontab line.

The raw box and the builder stay in sync both ways. Anything the builder cannot draw, such as 1-5,20, falls back to the list mode for that field instead of being lost.

Syntax cheat sheet

• * means every value. */15 means every 15th value starting at the minimum.
• 9-17 is a range. 10-40/10 is a range with a step, giving 10, 20, 30 and 40.
• 1,15 is a list. Lists can mix values and ranges: 1-5,20.
• Months and weekdays accept names: JAN-MAR, MON,WED,FRI.
• @daily, @hourly, @weekly, @monthly and @yearly are accepted as shortcuts.

Mistakes worth checking

• Day-of-month and weekday together. 0 9 1 * MON runs on the 1st and every Monday, not only on Mondays that are the 1st. To get the second case, schedule daily and test the date inside the job.
• Step does not reset. */7 in the minute field runs at 0, 7, 14 ... 56, then at 0 again, so the gap at the top of the hour is 4 minutes.
• Time zones. Cron daemons usually use the server's zone, often UTC. Choose the zone your server uses in the time-zone menu to see the times you will actually get.
• Overlapping runs. A job set to every minute that sometimes takes longer than a minute will pile up. Add a lock or use a longer interval.
• Impossible dates. 0 0 31 2 * is valid syntax but never fires; the tool tells you when no runs are found.

Using the output

The crontab line adds a placeholder command after the expression. Paste it into crontab -e, replace the path, and redirect output (>> /var/log/job.log 2>&1) so failures leave a trace.