Cron Debugger - debug any cron expression online

Common mistakes include using the wrong field count, exceeding field ranges, assuming day-field semantics are portable, treating steps as elapsed intervals, and overlooking timezone or DST behavior. Validate against the production scheduler after checking CronTool's preview.

The host may have been offline, the scheduler delayed work, or a concurrency policy may have suppressed a run. Vercel invocations can overlap; prevent that with an application lock. Quartz uses @DisallowConcurrentExecution for non-overlap behavior.

Compare run duration with schedule frequency, then inspect the scheduler's concurrency policy. CronTool shows intended start times but cannot tell whether a previous process is still running. Expression: */5 * * * *

Vixie cron ORs the restricted day fields, so it runs at 09:00 on every Monday and on the first of every month. Leave one day field unrestricted. Expression: 0 9 1 * 1

Vixie cron can skip a nonexistent spring-forward time and repeat a fall-back time. EventBridge Scheduled Rules use UTC; EventBridge Scheduler can use an IANA timezone. Check the target scheduler's DST policy. Expression: 30 2 * * *

Compare the timezone selected in CronTool with the scheduler's clock. Vercel uses UTC, GitHub Actions defaults to UTC, Kubernetes supports .spec.timeZone, and Vixie-compatible cron may support CRON_TZ.

Compare CronTool's five expected runs with actual scheduler logs. Then check the scheduler timezone, Vixie day-field OR behavior, daemon status, PATH, environment variables, working directory, permissions, and command output.

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.

Paste it into CronTool to validate fields, read the sentence, inspect five next runs, and navigate the month calendar. Test the command separately in cron's environment or use your scheduler's documented one-off mechanism.

Schedule validity only proves the clock rule parses. The daemon may be stopped, the command may fail, permissions may block it, or the host may be offline.

Cron usually has a shorter PATH and does not source interactive shell profiles. Use absolute executable paths and export every required environment variable explicitly.

Yes. In traditional crontab commands, an unescaped % may split the command and standard input, so escape literal percent signs or put the command in a script.

Cron does not guarantee the working directory your interactive shell uses. Change directory explicitly or use absolute paths for scripts, inputs, and logs.

Include cron=<expression>, mode=extended when needed, and tz=<IANA zone> when timezone matters. CronTool's built-in Copy link omits timezone, so add it before sharing a timezone-sensitive case.

Runtime failures remain: daemon state, PATH, environment, permissions, logs, exit status, and process overlap. CronTool never runs commands; its calendar isolates schedule mistakes.