Cron Expressions: Syntax, Examples, and Scheduling Notes

Cron expressions describe recurring times. The familiar five-field form is used by traditional Unix cron and many compatible schedulers, but providers add different fields, time-zone rules, and special characters. Treat an expression as belonging to a named implementation; verify it with that implementation before relying on it.

Read the five fields

A traditional five-field expression is:

minute hour day-of-month month day-of-week

The fields are separated by spaces. For example:

30 2 * * 1-5

On a conventional five-field cron, this means 02:30 on weekdays, using that scheduler's time zone. In a crontab file, the command follows the schedule:

30 2 * * 1-5 /usr/local/bin/backup.sh

The five fields alone are the schedule; the path and command are crontab syntax, not part of the cron expression. Use absolute paths because cron jobs often start with a limited environment and an unexpected working directory.

Build and check common schedules

The wildcard * means every valid value for that field. A comma lists values, a hyphen gives a range, and / gives a step. Examples:

Test the expression with the actual target scheduler or its official local simulator. A generic cron calculator may use different rules. There is no universal --test flag for cron libraries. For a Linux crontab, install it with crontab -e, list it with crontab -l, and inspect the command's own logs and exit behavior.

Day-of-month and day-of-week are a common trap

In many traditional Unix cron implementations, when both day-of-month and day-of-week are restricted (neither is *), a job runs when either field matches. For example:

0 9 1 * 1

may mean 09:00 on the first day of each month or every Monday, not only when the first falls on Monday. This behavior is implementation-specific. Quartz-style schedulers commonly use ? to leave one of those fields unspecified; do not copy that symbol into a five-field Unix crontab unless its implementation documents it.

Likewise, Sunday numbering and support for names or symbols such as L, W, and # differ. Check the target's syntax before using them. The Linux crontab manual describes the Linux field rules; Cloudflare Cron Triggers and AWS EventBridge Scheduler document their own formats.

Time zones and daylight saving

First decide whether the job should follow a wall clock (for example, 09:00 in London) or a fixed time worldwide (for example, 09:00 UTC). Then configure the scheduler explicitly where it supports a time zone:

• Linux cron variants may support a CRON_TZ setting in the crontab. TZ can instead affect the command's environment; it does not universally set the schedule's time zone.
• node-cron evaluates in the system time zone by default and accepts a timezone option, such as UTC or Europe/London.
• Cloudflare Workers Cron Triggers run on UTC.
• AWS EventBridge Scheduler accepts a time zone as part of the schedule configuration; its cron expression also has a year field and different day-field rules.

A local time can be skipped or repeated when daylight saving clocks change. The exact result depends on the scheduler. If a task must run exactly once at a fixed instant, prefer UTC or verify how the selected scheduler handles daylight-saving transitions. See the node-cron syntax and time-zone options before using its syntax.

Examples from different schedulers

Linux or macOS with traditional cron

Open the current user's crontab:

crontab -e

Add a command with an explicit executable path and redirect output if you need a simple log:

0 3 * * * /usr/bin/python3 /home/user/scripts/daily_report.py >> /home/user/logs/report.log 2>&1

This is 03:00 according to the cron daemon's time zone, not automatically UTC. Paths, shell and environment behavior can vary between systems.

Node.js with node-cron

node-cron supports five fields and an optional leading seconds field. Its seconds example is library syntax, not traditional crontab syntax:

import cron from 'node-cron'

cron.schedule(
  '0 9 * * 1-5',
  () => {
    console.log('Weekday task')
  },
  { timezone: 'UTC' }
)

This schedules the callback at 09:00 UTC on weekdays. It runs inside the Node.js process, so it will not run while that process is stopped. Use a process supervisor or managed scheduler when a job must survive application restarts.

Cloudflare Workers

Workers Cron Triggers use five fields and UTC. The schedule is attached to a Worker's scheduled() handler; it does not run a shell command. Follow the Workers documentation for configuration and local testing.

Troubleshoot deliberately

• Nothing runs: check the installed crontab, service status, executable permissions, absolute paths and command logs.
• It runs at the wrong local time: check the scheduler's configured zone and daylight-saving behavior; do not assume the shell's TZ changes the schedule.
• A month/day expression runs more often than expected: check how that implementation combines day-of-month and day-of-week.
• Runs overlap: cron usually starts a new invocation on schedule without knowing whether the previous one finished. Add a lock or use a scheduler with a documented no-overlap option if overlap would be harmful.
• A machine was off: traditional cron may miss that run. A catch-up service such as anacron has different behavior; consult the relevant system documentation.

A cron expression is only one part of reliable scheduling. Confirm the runtime, time zone, retry behavior and failure reporting for the system that will execute the work.