Cron Builder

A visual cron expression builder with 6 export formats. Free, open source, no signup, no tracking. Single HTML file.

🌐 Live at cron.parallelromb.dev

What it does

Build a cron expression three ways — visual builder, 22 presets, or paste & parse — then export it to whatever scheduler you actually use:

Export	What you get
Crontab	Standard 0 9 * * 1-5 line ready to paste into crontab -e
Launchd	Full macOS .plist with <key>StartCalendarInterval</key>
Systemd	.timer + .service unit files for Linux
GitHub Actions	on: schedule: workflow YAML (with the UTC reminder)
Kubernetes CronJob	apiVersion: batch/v1 manifest with timeZone field
node-cron	JS snippet with the right cron.schedule() call

Plus timezone-aware next-run preview — pick from 19 common timezones (UTC, browser local, all the America/*, Europe/*, Asia/*, Australia/*, Pacific/Auckland) and see the next 10 firings in that zone. This is the bit that catches people setting GitHub Actions schedules from non-UTC zones.

Why another cron tool?

• crontab.guru — lovely description engine, but no visual builder, no presets, no exports beyond crontab itself.
• Online-cron-job-builders — usually a web form that emits one syntax. Most don't help if your target is launchd, systemd, or Kubernetes.

This tool's moat is the 6-format export plus the timezone preview. If you're a polyglot dev who runs jobs across cron, GitHub Actions, and a Kubernetes cluster, this saves you man 5 crontab plus three doc-page tabs.

Features

• 🧱 Visual field builder (minute/hour/day/month/weekday) with hover hints showing syntax for each field
• ✅ Live validity badge + human-readable description
• 🕐 Next-10-runs preview with timezone selector (browser local / UTC / 17 named zones)
• 🎯 22 presets (every minute → quarterly)
• 📋 6 export formats (crontab · launchd · systemd · GitHub Actions · Kubernetes · node-cron)
• 🔗 Shareable URL — ?expr=0+9+*+*+1-5 opens directly with that expression loaded
• 💾 Save to library + auto-history (all in localStorage)
• 🌓 Dark mode (auto from prefers-color-scheme, toggle persisted)
• 📱 Mobile-responsive
• ♿ Keyboard navigation throughout

Run locally

git clone https://github.com/parallelromb/cron-builder.git
cd cron-builder
python3 -m http.server 8000
# open http://localhost:8000

Or just open index.html in your browser. It's a single self-contained HTML file with no build step.

Deploy

It's static. Drop index.html anywhere:

• Cloudflare Pages — connect the repo, leave build empty, output dir /
• GitHub Pages — Settings → Pages → Deploy from branch main, root /
• Vercel / Netlify — same, no build config needed

URL parameters

Param	Example	What it does
?expr=	?expr=0+9+*+*+1-5	Pre-loads this expression on page open

You can use + or %20 for spaces.

Privacy

Everything runs in your browser. No data leaves your machine — there's nothing to leave it for, since there's no backend. Your saved expressions and history live in your browser's localStorage for this domain only.

Contributing

PRs and issues welcome. Some directions worth taking:

• More export formats (Quartz, AWS EventBridge, Airflow, BullMQ)
• Reverse parser — paste a launchd .plist and get the cron expression back
• Quartz-style 6-7 field expressions (with ?, L, W, # operators)
• More timezones / IANA TZ search
• Translations

Big rewrites or framework migrations: please open an issue first.

License

MIT — fork, embed, sell, whatever. Just keep the copyright line.

About

Built by Sri (parallelromb.dev) — solo hacker building Lumen, Smara, MCP Doctor, Scoper, and a few other things.

Originally part of parallelromb/lumen-tools — a marathon-night build of 58 self-contained browser tools — extracted and polished as a standalone product.

Pair-programmed with Claude (Anthropic).