Cron Expression Explainer

A standard crontab line uses five fields in order: minute (0–59), hour (0–23), day of month (1–31), month (1–12), and day of week (0–6, where 0 is Sunday). Together they answer “at which minutes, of which hours, on which calendar days/months/weekdays should this job run?” This tool targets that classic five-field format used by Linux crontab and many cloud schedulers.

* means “every valid value” for that field. A comma lists discrete values (e.g. 1,15). A dash sets a range (e.g. 9-17). A slash sets a step (e.g. */5 in minutes means every 5 minutes; 0-30/10 means 0, 10, 20, 30). You can combine them, such as 1,15,30 or MON-FRI.

Common pitfalls include mixing up day-of-week numbering (Sunday as 0 vs 1), forgetting that when both day-of-month and day-of-week are restricted they usually combine with OR (not AND), using a six-field “seconds” expression in a five-field crontab, and assuming the scheduler’s timezone matches your laptop. Always check the next few run times in the timezone the job will actually use.

Production schedules often encode internal maintenance windows, billing cutoffs, or batch cadences you may not want sitting in a third-party log. This explainer parses, describes, and projects next run times entirely in your browser — nothing is uploaded — so you can safely validate infrastructure schedules without leaking them to an external service.