Cron Expression Generator

What this tool does

Cron's five fields are easy to write and hard to be sure about. Set each one, or start from a schedule that already does roughly what you want, and the page shows the two things that tell you whether the expression is right: a sentence describing it and the next ten moments it would fire. If the tenth run is not where you expected, the expression is wrong, whatever it looks like.

The optional seconds and year fields are for Quartz, Spring's @Scheduled and node-cron. Leave them off for anything going into a real crontab, a Kubernetes CronJob or a GitHub Actions schedule, none of which accept them.

Common uses

• Writing the schedule for a nightly backup, a cache warm or a report job.
• Spreading load: turning "every hour" into 17 * * * * so a fleet of machines does not all fire on the minute.
• Producing the spec.schedule value for a Kubernetes CronJob and confirming the cadence before applying it.
• Checking that a business-hours job really does skip the weekend.

A short example

Quarter-hourly during office hours on weekdays:

*/15 9-17 * * 1-5

Every 15 minutes between 09:00 and 17:59, on Monday through Friday.
That is 36 runs a day, 180 a week, and the last one is at 17:45.

Note that 9-17 includes hour 17 in full, so the schedule runs until 17:45, not 17:00. For a last run at 17:00 exactly, add 0 17 * * 1-5 as a second entry rather than trying to express it in one.

Writing each field

Every field takes the same four constructions, which combine freely with commas: a single value (5), a range (9-17), a step (*/15, or 9-17/2 for every second hour inside a range) and a list (1,15,30). Month and day-of-week additionally accept three-letter names, so MON-FRI and 1-5 are identical, as are JAN and 1.

Steps are the most misread. */20 in the minute field means minutes 0, 20 and 40 — it counts from the start of the range, not from the moment you install the job, so it does not mean "every twenty minutes from now". A step larger than the field's range can only ever match its first value, so this tool refuses it. Sunday is both 0 and 7; February 30th is accepted by the syntax and simply never fires, which the tool warns about.

Worth knowing

A crontab entry inherits a minimal environment: typically PATH=/usr/bin:/bin, no shell profile, and HOME from the passwd file. Commands that work in your terminal frequently fail under cron for this reason alone, so use absolute paths. Percent signs are special in a crontab — they become newlines fed to the command's standard input unless escaped as \%, which catches out anyone using date +%Y-%m-%d in a job. And output is not discarded: anything written to stdout or stderr is mailed to the owner, or silently dropped if no mailer exists.