Cron Expression Parser

How this calculation works

Each of the five fields is expanded into the set of values it matches, then the parser walks forward minute by minute to find the next times every field is satisfied at once. Walking is deliberate rather than lazy: cron treats the two day fields as OR when both are restricted, and every closed-form shortcut has to special-case that anyway. The scan is bounded to four years, so an expression that can never fire reports as much instead of hanging.

What the results mean

In English

The schedule described in words. Reading this back is the quickest way to catch a field in the wrong position, which is the most common cron mistake after the day trap.

Day fields

Flagged when both day of month and day of week are restricted, because cron then runs when either matches rather than both.

Runs per day

How many times the job fires on a day the date fields allow. Useful for spotting an expression that runs far more often than intended.

Common problems and fixes

The job runs far more often than expected

Check for a step in the wrong field. Writing */5 in the hour position means every five hours, while the same thing in the minute position means twelve times an hour. Also check for a bare number where you meant a step: 5 in the minute field means five past every hour, which is 24 runs a day rather than one.

A job scheduled for the 1st and for Mondays runs on both

That is cron behaving to specification, and it surprises almost everyone. When day of month and day of week are both restricted, the job runs when either matches, not when both do. To get the intersection, restrict only the day of month and have the script itself check the weekday before doing anything.

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