무료 cron expression 디코더를 만들었습니다, 내부 작동 방식

Source: Dev.to

The problem

모든 개발자는 한 번쯤은 겪어봤을 겁니다: 설정 파일에 있는 0 */4 * * 1-5 같은 난해한 크론 표현식을 바라보는 상황을. 이를 해독하려다 지쳐서, 저는 Cron.Explain이라는 무료 도구를 만들었습니다. 이 도구는 어떤 크론 표현식이든 즉시 평문 영어로 해석해 줍니다.

Features

• Plain English explanation – e.g. 0 9 * * 1-5 → “At 9:00 AM, on Monday through Friday”.
• Field‑by‑field breakdown of the five cron parts.
• Next 5 run times calculated from the current moment.
• Free REST API – no API key required.

Usage

Web tool

Paste any cron expression at the live tool:
https://timely-flan-0ca3c1.netlify.app

REST API

curl -X POST https://timely-flan-0ca3c1.netlify.app/api/explain \
  -H "Content-Type: application/json" \
  -d '{"cron": "0 9 * * 1-5"}'

Response

{
  "expression": "0 9 * * 1-5",
  "explanation": "At 9:00 AM, on Monday through Friday",
  "fields": { /* detailed breakdown */ },
  "nextRuns": [
    "2026-02-23T09:00:00.000Z",
    "2026-02-24T09:00:00.000Z",
    "2026-02-25T09:00:00.000Z",
    "2026-02-26T09:00:00.000Z",
    "2026-02-27T09:00:00.000Z"
  ]
}

Implementation details

• Parser – No external cron‑parsing libraries. The core is a matchesField(value, n, min) function that handles all special cron characters (*, */n, n-m, n,m, n/m).
• Next‑run calculator – Starts from the current minute and increments forward, checking each minute against all five fields until it finds five matches. It caps at 500 000 iterations to avoid infinite loops on extremely rare schedules.
• Tech stack
  • Frontend: React + Vite
  • Serverless API: Netlify Functions
  • Zero dependencies for the parser itself
  • Font: IBM Plex Mono (chosen for a developer‑friendly look)

Future plans

• Cron job monitoring – Ping a URL and alert if a scheduled job hasn’t run.
• Reverse builder – Input an English description and receive the corresponding cron expression.
• Additional languages in the API documentation.

Source code & documentation

• GitHub repository – open source, contributions welcome.
• Live tool:
• API docs: