Cron Expression Explainer

How it is calculated

The expression must have exactly five fields separated by spaces, in this order: minute hour day-of-month month day-of-week. Each field is checked against its range (minute 0-59, hour 0-23, day 1-31, month 1-12 or jan-dec, weekday 0-7 or sun-sat), and steps (*/15), ranges (9-17) and comma lists (1,3,5) are all understood. Any field that is out of range, empty or malformed, and any shortcut such as @daily, gives no result. The tool then builds one sentence: a time (At 02:30, or Every minute), then the day of month, month names and weekday names in the page language. When both day fields are restricted, standard cron runs the job if either one matches, and the sentence says so. The breakdown lists the five fields as typed. The tool does not check whether a date exists, so day 31 in February still gets a sentence.

Common mistakes

• Setting both day-of-month and day-of-week, as in 0 0 1 * 1. It reads as if both must be true, but standard cron runs the job when either one matches, so it fires on the 1st and on every Monday. The tool adds a note to the sentence when it sees this.
• Writing */15 in the hour field when you mean every 15 minutes. The field order matters: */15 * * * * is every 15 minutes, while 0 */15 * * * is every 15 hours, on the hour.
• Pasting a six-field expression with seconds, a Quartz-style ? or a shortcut such as @daily. Only five standard fields are accepted, and anything else returns no result.
• Treating the sentence as proof the job will ever run. A date such as 0 0 31 2 * (31 February) is described normally although it can never occur.
• Confusing hour 24 with midnight. Hours run from 0 to 23, so 0 24 * * * is rejected; use 0 0 * * * for midnight.