Cron Expressions Explained: A Beginner's Guide

A cron expression is a five-field pattern that tells a scheduler exactly when to run a job — used in Linux cron jobs, CI/CD pipelines, and countless scheduling tools. The syntax looks dense at first, but it's really just five fixed positions, each with its own meaning.

The five fields

• Minute (0–59)
• Hour (0–23)
• Day of month (1–31)
• Month (1–12)
• Day of week (0–6, where 0 is Sunday)

In order, that's: `minute hour day-of-month month day-of-week`. A `*` in any field means "every value" for that position.

Real examples

• `0 * * * *` — every hour, on the hour.
• `0 0 * * *` — once a day, at midnight.
• `*/15 * * * *` — every 15 minutes.
• `0 9 * * 1-5` — 9 AM, Monday through Friday only.
• `0 0 1 * *` — midnight on the 1st of every month.

Step-by-step: build a cron expression

1. Open the Cron Expression Helper.
2. Pick or type a schedule — the tool explains in plain English what it means as you build it.
3. Check the next few run times it predicts to confirm it matches what you expect.
4. Copy the expression into your scheduler, CI config, or cron job.

Common mistakes

• Confusing day-of-month and day-of-week fields — both can technically constrain the same run, and combining them without understanding the AND/OR interaction (most cron implementations treat them as OR when both are restricted) leads to unexpected extra runs.
• Forgetting that `*/15` style step values only start counting from the field's minimum (0), not from whatever time you deployed the job.
• Not accounting for server timezone, leading to jobs running at 2 AM local time instead of the intended hour.