CronTool
Cron expression editor & debugger

Cron Debugger — debug any cron expression online

Paste a cron expression that's misbehaving and CronTool tells you exactly when it will fire next, what each field means, and which validation rule it's breaking. The debugger works for standard Unix crontab, AWS EventBridge / Lambda Scheduled, Vercel cron jobs, Quartz Scheduler and other extended cron implementations.
The most common cron bugs are not syntax errors — they're schedule mismatches. Seeing the next 30 runs on a real calendar makes the wrong hour, the missed weekend, or the day-of-week vs day-of-month conflict obvious in seconds.

Examples

  • 018***
    Every day at 18:00
  • 0*/5***
    Every 5 hours
  • 018**1-5
    Weekdays at 18:00
  • 001**
    Once a month

Cheatsheet

FieldRequiredValues RangeWildcardsminuteYes0-59, - * / hourYes0-59, - * / day of monthYes1-31, - * / L W monthYes1-12, - * /day of weekYes0-7, - * / L

Calendar

View future cron matches in a calendar

April 2026

Showing next 1000 cron schedules

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How to debug a cron expression

Drop your expression into the field grid above. Every invalid or out-of-range value is highlighted in red, with the parser error explained in plain English at the bottom. The calendar instantly redraws to show when the cron really fires — including weekends, holidays and the awkward end-of-month cases.

Toggle the extended cron expressions switch in the top bar to enable the seconds and year fields used by Quartz, AWS EventBridge (6 fields) and Vercel cron jobs. Switching modes also activates the ?, L and W modifiers.

Common cron bugs the debugger catches

  • Setting both day-of-month and day-of-week in standard cron (which OR-joins them and produces unexpectedly frequent runs).
  • Using ? in standard crontab (only valid in Quartz and a handful of other extended dialects).
  • Mixing the seconds field in (and out) by accident — most systems have 5 fields, AWS EventBridge has 6, Quartz has 6 or 7. The debugger flags the mismatch.
  • */7 in a 24-hour clock — fires every 7 hours starting at 00 but doesn't hit 24:00 (it wraps).
  • 0 vs 7 for Sunday differs across schedulers; CronTool normalises both, but warns you when using Quartz syntax.

Share debugging context with your team

Use the Get link button to copy a shareable URL with the cron pre-filled. Drop it in the PR description, the runbook or the Slack thread — whoever opens it sees the same calendar and validation output you did. No login, no account, no paid plan.

For multi-cron debugging (e.g. a job that runs every 15 minutes except 03:00), use the multi-cron visualizer — it overlays multiple expressions on one calendar with color-coded labels.

Tired of debugging cron?

Crontap schedules API calls on cron without you maintaining a daemon, a container, or a shell user. Bring your endpoint, drop a cron, and watch it run.

Frequently asked questions

What are the most common cron expression mistakes?

Setting both day-of-month and day-of-week to non-wildcards (the standard cron OR-joins them, while Quartz forbids it). Forgetting that hours are 0-23 (not 1-24). Assuming `*/7` divides 24 evenly when running every 7 hours. Confusing `0` and `7` for Sunday in different schedulers. Running every minute when you really want every minute *of the hour*.

Why is my cron not running when I expect?

First, double-check the timezone — cron runs in the system timezone of the machine (or UTC for AWS / Vercel). Second, check the day-of-month vs day-of-week conflict (see above). Third, ensure the user's PATH and environment variables are set; many cron failures come from `command not found` because login profiles aren't sourced. Fourth, look at the actual schedule on a calendar — that's exactly what CronTool's debugger shows you.

How do I capture cron output for debugging?

Redirect both stdout and stderr in your crontab line: `* * * * * /your/script.sh >> /var/log/myjob.log 2>&1`. Without redirection, output is mailed to the user account, which you may not check. For containerised cron (Kubernetes CronJobs, AWS Lambda Scheduled), check the platform's log stream.

How can I test a cron expression without waiting for it to fire?

Paste the expression into CronTool — the debugger immediately shows the next 30+ runs on a calendar, validates the syntax, and decodes each field. To test the underlying script, run it manually with `bash -lc /your/script.sh` or trigger a one-off invocation in your scheduler (AWS EventBridge has a 'send test event', Kubernetes has `create job --from=cronjob`).

Why does my cron skip runs?

Most often: previous run is still executing when the next is due, and your scheduler refuses overlap (Vercel cron, Quartz with `concurrent: false`, AWS EventBridge with concurrency limits). Sometimes the host was offline at the scheduled minute. Use a wrapper that tracks last-run timestamp and reschedules missed runs if you need at-least-once semantics.

How does cron handle daylight saving time changes?

Standard Unix cron honours the system timezone, so jobs scheduled in the skipped hour during the spring transition can be skipped, and jobs in the repeated hour during autumn can fire twice. AWS EventBridge runs in UTC unless you set a timezone explicitly. The safest pattern for daily jobs that must run exactly once: schedule in UTC.

Ready to schedule it?

Point Crontap at any URL. Pick any cron. Done.

WordPress, Shopify, Railway, Cloud Run, Vercel, HubSpot, Ghost, your own box. If it answers HTTP, Crontap can drive it on a clock you can read, in the timezone that actually matters, and page you when something breaks.

Get startedSee examples

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Your next schedule
GET/wp-cron.php?doing_wp_cron=1

Schedule

“every 5 minutes”

Next run

in 23s

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