What is Cron?

What it is: A small text format for telling a computer when to run a task — every Monday at 9am, every five minutes, the first of every month.
Who it’s for: Anyone who wants something to happen on a schedule without being there to start it.
Best if: You have repeating work, or a future check-in you want to fire automatically.
Skip if: You only need a one-off reminder — your calendar handles that.

Cron is older than most software you’ve used. It’s been the way Unix computers schedule recurring jobs since the 1970s. The name comes from chronos, the Greek word for time.

Despite the age, you’ll see cron everywhere modern tools schedule themselves — including AI agents like Claude Code when they use the ScheduleWakeup feature to fire future check-ins. Knowing what a cron expression is means you can read what your AI just scheduled.

What a cron expression looks like

A cron expression is five fields separated by spaces:

* * * * *
| | | | |
| | | | └─ day of the week (0–6, Sunday is 0)
| | | └── month (1–12)
| | └─── day of the month (1–31)
| └──── hour (0–23)
└───── minute (0–59)

A * means “any value.” So:

• 0 9 * * 1-5 → 9:00am every weekday
• 0 0 1 * * → midnight on the 1st of every month
• */15 * * * * → every 15 minutes

That’s the whole language. You almost never need more than three or four examples to handle real-world schedules.

Why it shows up in AI tools

When an AI agent can schedule itself — “check on this build in eight minutes,” “run a sweep every Monday at 11” — what it’s writing under the hood is a cron expression. The format is universal, so the same schedule works whether the agent runs on your laptop, on a server, or in the cloud.

You don’t have to write cron yourself. You describe what you want in plain English and the AI translates. But knowing the format exists means you can verify what got scheduled.

Related

• What is ScheduleWakeup?
• What is Claude Code?