Crontab Calculator

Type a cron expression and see what it means in plain English, along with the next five times it will run.

The five fields

A standard crontab entry has five fields followed by the command:

┌───────── minute        (0-59)
│ ┌─────── hour          (0-23)
│ │ ┌───── day of month  (1-31)
│ │ │ ┌─── month         (1-12)
│ │ │ │ ┌─ day of week   (0-6, Sunday is 0)
│ │ │ │ │
* * * * * command_to_run

Each field accepts a single value, a range (9-17), a step (*/15), a list (1,15), or any combination. A few keywords are also understood: @hourly, @daily, @weekly, @monthly, @yearly and @reboot.

The day-of-month trap

This is the one that catches people out, and most calculators do not mention it. If you set both the day-of-month field and the day-of-week field to anything other than *, cron does not require both to match — it runs when either matches.

So 0 0 13 * 5 reads as “midnight on Friday”, but it actually means “midnight on the 13th, or on any Friday”. If you want the 13th only when it falls on a Friday, you cannot express that in a single crontab line — add a date check inside the command instead. The calculator flags the case whenever it applies.

Other things that go wrong

• Timezones. Cron runs in the server’s local time, not yours. The runs below are shown in this device’s timezone so you can see the difference — if the server is set to UTC and you are not, a job scheduled for 02:00 may not run when you expect.
• Environment. Cron jobs get a minimal environment and often a different PATH than your shell. A script that works when you run it by hand and fails under cron is almost always this.
• Overlap. If a job takes longer than its interval, the next run starts before the previous one has finished. For anything doing real work, add a lock file.
• Silent failure. Cron mails output to the local mailbox, which nobody reads. Redirect output somewhere you will actually see it, or you will not know a backup stopped running six months ago.

Verify the schedule, not the syntax

A cron line can be perfectly valid and still not do what you meant. The next-five-runs list is the real check: if you expected a weekly job and you are seeing daily runs, you have your answer before you ever hit save.