# Cron expression explainer

> Paste a crontab schedule and get every field read back in plain language, the footguns flagged, and the next runs computed.

- Tool: https://ronutz.com/en/tools/cron-expression-explainer
- Family: Text & utilities

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# Cron expression explainer

Paste a five-field crontab schedule — or one of the `@` macros — and the tool reads it back: each field explained in plain language with the exact set of values it matches, the next five occurrences, and the dialect's footguns flagged as you type.

## The five fields

`minute hour day-of-month month day-of-week`, with ranges `a-b`, lists `a,b`, steps `*/n` and `a-b/n`, and names (`JAN`–`DEC`, `SUN`–`SAT`). The `@` macros — `@yearly`, `@monthly`, `@weekly`, `@daily`, `@hourly` and friends — expand to their five-field forms; `@reboot` has no schedule at all, and the tool says so instead of inventing one.

## The footguns it flags

The big one: when day-of-month **and** day-of-week are both restricted, cron runs the command when **either** matches — an OR, not the AND almost everyone expects. Also: `0` and `7` are both Sunday; a step that does not divide its span evenly wraps ragged (`*/7` in minutes ends at 56, then jumps to 0); names inside stepped ranges are historically unreliable across implementations; and a six-field input is recognized as Quartz's seconds dialect and named as such.

## About the projected times

cron evaluates schedules in the daemon's **local** time. The occurrences here are computed on the wall-clock of your browser's current instant, with no time-zone database applied — the same deterministic engine the golden vectors pin against a frozen reference moment.

Everything runs locally; the schedule never leaves the page. Grounded in `crontab(5)` and the POSIX specification.

## Standards and references

- [crontab(5), man7.org - the five fields and their ranges, lists/ranges/steps, month and weekday names, 0 and 7 both meaning Sunday, the day-of-month OR day-of-week rule, and the @-macro table](https://man7.org/linux/man-pages/man5/crontab.5.html) - the whole field grammar, the OR rule, and every @macro expansion
- [POSIX crontab (The Open Group Base Specifications) - the portable core of the format the Vixie extensions build on](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/crontab.html) - the baseline five-field grammar and field domains

## Related reading

- [cron: five fields that run the world's chores](https://ronutz.com/en/learn/cron-the-schedule-language.md): Every Unix box carries a scheduler whose entire language is five columns of numbers and stars. How to read them, the OR rule that catches almost everyone, why 0 and 7 are the same day, what the @ macros expand to, and the local-time reality behind every projected run.