The problem

Cron syntax is notoriously hard to verify — developers copy expressions from Stack Overflow or write their own, then paste them into random online parsers to check them, often without offline access or confidence in edge-case handling.

If you've hit this before, you know how it goes — you fire up a browser tab, paste the expression into crontab.guru, and hope it interprets edge cases the same way your cron daemon does.

As a solution, I created croncheck

Parse and preview cron expressions interactively — see the next 10 run times before you deploy

It's zero-dependency Node.js, so you can run it immediately without installing anything:

npx croncheck

Output:

croncheck "30 9 * * 1-5"

  Expression: 30 9 * * 1-5
  Meaning:    minute 30, hour 9, every day, every month, weekdays 1-5

  Next 10 run times:
   1. Mon May 25 2026  09:30  in 4d 23h
   2. Tue May 26 2026  09:30  in 5d 23h
   3. Wed May 27 2026  09:30  in 6d 23h
   4. Thu May 28 2026  09:30  in 7d 23h
   5. Fri May 29 2026  09:30  in 8d 23h
   6. Mon Jun  1 2026  09:30  in 11d 23h
   7. Tue Jun  2 2026  09:30  in 12d 23h
   8. Wed Jun  3 2026  09:30  in 13d 23h
   9. Thu Jun  4 2026  09:30  in 14d 23h
  10. Fri Jun  5 2026  09:30  in 15d 23h

How it works

Pure Node.js cron parser that expands each field (minute, hour, day, month, weekday) into a sorted array of valid values using set arithmetic, then advances a Date object forward one minute at a time to collect the next N matching timestamps, rendered in a raw-mode terminal UI.

Why I built it

Found repeated complaints on r/devops and HN from developers who deployed broken cron schedules after writing expressions from memory. Online tools like crontab.guru exist but require a browser and fail offline. No popular zero-dep npx tool fills the gap for quick in-terminal verification before committing a schedule to production.

Try it

npx croncheck --help

Part of µ micro — one new developer tool, shipped every day. All tools are zero-dependency Node.js and run instantly with npx.