Cron式ジェネレーター

You are an experienced full-stack web developer. Build a web tool called "Cron式ジェネレーター" that meets the requirements below.

## Goal

Build a tool that creates and explains standard five-field cron expressions from a chosen schedule.

## Core features

- Schedule picker (every N minutes, hourly, daily, weekly, monthly)
- Plain-language description of the expression
- Preview of the next five run times
- Time zone selection (UTC, Asia/Seoul, …)
- Parse an expression typed by the user

## Inputs

- Schedule type and minute/hour/weekday/day values
- Time zone

## Outputs

- Cron expression
- Description
- Next run times

## Edge cases and handling rules

- Warn that setting both day fields means OR
- Explain steps that do not divide evenly, such as */45
- Validate out-of-range values like minute 60
- Explain run-time differences on UTC servers

## Implementation requirements

- Build it as a single page (HTML, CSS, JavaScript) that runs entirely in the browser, and never send the user's input to a server.
- Make the UI responsive and easy to use on mobile; label every input and keep everything keyboard accessible.
- Add a Copy button for results and show clear, friendly error messages for invalid input.
- Keep the core logic in functions separate from the UI so it can be unit-tested.

## Definition of done (test cases)

- Daily at 02:00 gives '0 2 * * *'
- Weekdays at 09:30 give '30 9 * * 1-5'
- '*/15 * * * *' runs every 15 minutes

First outline your implementation plan in at most five lines, then write the complete code, and finally explain how to check the test cases above.

---
name: build-cron-generator
description: Use when building or changing a Cron式ジェネレーター tool. Build a tool that creates and explains standard five-field cron expressions from a chosen schedule.
---

# Build: Cron式ジェネレーター

## Goal

Build a tool that creates and explains standard five-field cron expressions from a chosen schedule.

## Workflow

1. Review the required features, inputs and outputs below and ask about anything missing first.
2. Implement the core logic as pure functions separate from the UI.
3. Check the logic against the test cases in the verification list.
4. Build a responsive UI with an input form, a result area and a Copy button.
5. Reproduce each edge case listed under Rules and refine the error messages.

## Core features

- Schedule picker (every N minutes, hourly, daily, weekly, monthly)
- Plain-language description of the expression
- Preview of the next five run times
- Time zone selection (UTC, Asia/Seoul, …)
- Parse an expression typed by the user

## Inputs

- Schedule type and minute/hour/weekday/day values
- Time zone

## Outputs

- Cron expression
- Description
- Next run times

## Rules

- Warn that setting both day fields means OR
- Explain steps that do not divide evenly, such as */45
- Validate out-of-range values like minute 60
- Explain run-time differences on UTC servers
- Build it as a single page (HTML, CSS, JavaScript) that runs entirely in the browser, and never send the user's input to a server.

## Verification

- [ ] Daily at 02:00 gives '0 2 * * *'
- [ ] Weekdays at 09:30 give '30 9 * * 1-5'
- [ ] '*/15 * * * *' runs every 15 minutes