Cron Expression Explainer

A cron expression is a short string that tells a scheduler when to run a job: backups every night, reports every Monday, a cache refresh every 15 minutes. It comes from the Unix cron daemon and is used by crontab, Kubernetes CronJobs, GitHub Actions, Vercel and most cloud schedulers.

Its terse syntax is easy to misread. This explainer translates any 5-field expression or @-shortcut into English, shows the value of each field, flags errors such as an hour of 25, and computes the next five times it will fire, so you can confirm a schedule before deploying it.

Cron fields are minute (0–59), hour (0–23), day of month (1–31), month (1–12 or JAN–DEC) and day of week (0–6 or SUN–SAT, with 7 also meaning Sunday). Each can be *, a number, a list (1,15), a range (1-5) or a step (*/15, every 15).

One rule trips people up: when both day of month and day of week are restricted, standard cron runs on either, not both. 0 0 1 * 1 means the 1st of every month and every Monday. Servers usually run cron in UTC, so check the server's time zone.