Cron expression explainer

The short answer#

• A cron expression has five fields: minute, hour, day of month, month and day of week.

• 0 2 * * * means every day at 02:00; */15 * * * * means every 15 minutes.

• Run times depend on the time zone the scheduler uses; most servers default to UTC.

• When both day fields are restricted, the job runs when either matches, not both.

The five fields#

* means every value, a-b a range, a,b a list and */n every n-th value. This tool accepts exactly five fields; the six-field form with seconds and words like @daily are not accepted, because Belvador monitors do not accept them either.

Time zones and clock changes#

A job at 30 2 * * * in Europe/Berlin does not run on the night the clocks skip 02:30; Belvador's monitors expect it an hour later instead, at 03:30 summer time, and run a repeated hour once. The tool at the top of this page shows exactly that on those dates. Pick the time zone your scheduler uses: GitHub Actions and most Linux servers use UTC unless configured otherwise.

Common mistakes#

• Day of week 7. Some crons accept 7 for Sunday; use 0 to be portable.

• Both day fields. 0 9 1 * 1 runs on the first of the month and on every Monday.

• Every minute by accident. * 2 * * * runs 60 times between 02:00 and 02:59. You probably meant 0 2 * * *.

Watch the job, not only the schedule#

A correct expression does not tell you the job ran. A Belvador monitor with the same schedule opens an incident when a run fails or never arrives (cron monitoring, ping URL setup).