Cron Expression Explainer

How to use the cron expression explainer

1. Paste the cron expression.
2. Read the summary.
3. Check each field in the table below.

About this tool

Five fields — minute, hour, day of month, month, day of week — and each is explained separately here alongside a plain summary of the whole.

One behaviour is worth knowing because it silently does the wrong thing. When both day of month and day of week are set, cron combines them with OR, not AND. <code>0 0 1 * 1</code> does not mean &ldquo;the first of the month, if it is a Monday&rdquo;; it means &ldquo;the first of the month, and also every Monday&rdquo;. That is rarely what anyone intends, so it is flagged whenever both fields are used.

The six-field form with seconds is a Quartz and Spring extension, not standard cron, and is rejected by name rather than misread as a five-field expression with an extra field.

Cron runs in the server's timezone, not yours — the usual explanation for a job that fires an hour out after a daylight saving change.

Common uses

• Understanding a schedule found in an existing crontab.
• Checking an expression before deploying it.
• Learning cron syntax from working examples.

Frequently asked questions

What does */15 mean?

Every fifteenth value of that field. In the minute position it means every fifteen minutes.

Why does my job run more often than expected?

Probably both day of month and day of week are set. Cron treats them as OR, so the job runs when either matches.

Why is my six-field expression rejected?

Six fields with seconds is a Quartz and Spring extension rather than standard cron. This reads the standard five-field format.