Cron expressions explained (Unix five fields)

A Unix cron expression is five fields: minute, hour, day of month, month, day of week. Some systems add a seconds field in front. The meaning of a star, a list, and a step is easy to skim past and get wrong.

30 9 * * 1-5

That is 09:30 on Monday through Friday in the timezone the scheduler actually uses.

Rules that bite

• 0 and 7 are both Sunday in many crontabs
• Day-of-month and day-of-week together usually mean “either”, not “both”
• Quartz characters L, W, and # are not Unix cron
• GitHub Actions and Kubernetes may run in UTC even if you think in local time

Alphzen’s Cron Expression Generator explains a 5- or 6-field expression and previews the next fires in your browser timezone. Confirm on the system that will run the job.