5 Cron Job Mistakes That Will Take Down Your Production Server

Cron jobs that run in the wrong timezone, overlap, or fail silently can take down production or cause data corruption. Here are five common mistakes and how to avoid them—plus how to validate your cron expressions before they go live.

1. Ignoring Timezone

Cron runs in the server’s timezone, not yours. If the box is in UTC and you wrote 0 9 * * * thinking "9 AM local," that's 9 AM UTC. Deploy the same crontab to a server in another region and the same expression runs at a different local time.

Fix: Set TZ in the crontab or at the top of the script, or use a scheduler that supports timezone-aware schedules. Always document which timezone the schedule uses.

TZ=America/New_York
0 9 * * * /app/scripts/daily-report.sh

Validate what "next run" means: use a Cron Parser to see the next few run times for your expression and confirm they match what you expect.

2. No Error Handling or Logging

If a cron job fails, the default is often silence. No log, no alert, no retry. You only notice when data is missing or a downstream system breaks.

Fix: Redirect stderr and stdout to a log file, and exit with a non-zero code on failure so your monitoring can detect it. Consider a wrapper that logs start/end and exit code.

0 * * * * /app/scripts/sync.sh >> /var/log/cron/sync.log 2>&1 || echo "sync failed" | mail -s "Cron failed" ops@example.com

Even better: use a job runner or monitoring that alerts on failure so you don't rely on cron email.

3. Overlapping Runs

A job that runs every 5 minutes but sometimes takes 10 minutes will overlap. Two instances can double-process data, lock the same resource, or corrupt state.

Fix: Use a lock (e.g. flock) so only one instance runs at a time, or design the job to be idempotent and safe if run concurrently. For long-running work, consider a queue or scheduler that enforces single-run semantics.

0/5 * * * * flock -n /tmp/sync.lock /app/scripts/sync.sh

4. Wrong Cron Syntax (Off-by-One, Typos)

Cron fields are easy to get wrong: 0–6 for Sunday–Saturday vs 1–7, 0 vs 1 for the first day of the month, and typos like * * * * (four fields instead of five). A wrong expression can run at the wrong time or not at all.

Fix: Before deploying, validate the expression and check "next run" times. Use an online Cron Parser that runs in your browser so your expressions never leave your machine—no need to paste production schedules into random third-party servers.

5. No Visibility Into When Jobs Run

If you don't know when a cron actually fired, debugging "why did this run at 3 AM?" or "why didn't it run?" is guesswork. That wastes time and can hide production issues.

Fix: Log job start and end (and exit code). Use a parser to see the next N run times for each expression. Keep a small runbook or dashboard of what's scheduled and where it's defined so you can confirm behavior without SSH'ing into boxes.

Validate Cron Expressions Safely

Cron expressions are sensitive: they describe when production jobs run. Pasting them into unknown websites can leak schedule details or internal conventions. Our Cron Parser runs entirely in your browser: you get the next run times and validation without sending your expressions to our servers. Same idea as our JWT Decoder—local-first, no server round-trip.

Try it: Use our Cron Parser to validate your expressions and see the next run times before you deploy. For more on how we handle data, see our Privacy Policy.