Cron Expression Builder

Reading and writing cron schedules

Cron is the scheduling language that decides when a recurring job runs - backups, report emails, cache clears, you name it. A standard expression is five fields separated by spaces, and once you can read those five fields, the cryptic strings in your server configuration stop being intimidating.

The five fields

┌── minute (0–59)
│ ┌── hour (0–23)
│ │ ┌── day of month (1–31)
│ │ │ ┌── month (1–12)
│ │ │ │ ┌── day of week (0–6, Sun=0)
│ │ │ │ │
* * * * *

The special characters

• * - every value of that field.
• , - a list, e.g. 1,15,30.
• - - a range, e.g. 1-5 (Monday to Friday).
• / - a step, e.g. */15 means every 15 units.

0 9 * * 1-5     every weekday at 09:00
*/15 * * * *    every 15 minutes
0 0 1 * *       midnight on the 1st of each month
30 2 * * 0      02:30 every Sunday

The traps that bite

Two gotchas catch almost everyone. First, when you set both day-of-month and day-of-week, most cron implementations treat it as OR, not AND - the job runs if either matches, which is rarely what people expect. Second, cron runs in the server's timezone, so a job set for 09:00 may fire at a different local hour than you assumed. This builder previews the next run dates so you can confirm the schedule does what you intended before you commit it.