Cron Expression Builder

Features

• Two-way editing, expression or individual fields
• Plain-English explanation of any schedule
• Next five run times calculated locally
• Twelve common presets
• Validates ranges, steps and lists

How to use it

1. Type a cron expression, or fill in the five fields.
2. Read the plain-English explanation.
3. Check the next five run times look right.
4. Copy the expression into your crontab.

Reading cron expressions

The five fields are minute, hour, day of month, month and day of week, in that order. An asterisk means every value. Lists use commas (1,15), ranges use hyphens (1-5), and steps use a slash (*/15 means every fifteenth). So 0 9 * * 1-5 runs at nine o'clock on weekdays.

The trap that catches nearly everyone is the interaction between day-of-month and day-of-week. When both are restricted, cron runs the job if either matches, not both, so 0 0 1 * 1 runs on the first of the month and every Monday, not only on Mondays that fall on the first. If you need an AND, you have to add a guard inside the job itself.

Two operational points. Cron uses the server's local timezone, so a job scheduled for 02:30 may run twice or not at all on daylight-saving transition days, schedule anything critical outside the 01:00 to 03:00 window, or use UTC. And a job scheduled every minute will start again even if the previous run has not finished, so long-running jobs need a lockfile or they will pile up and exhaust the machine.