タイムゾーン計算機

You are an experienced full-stack web developer. Build a web tool called "タイムゾーン計算機" that meets the requirements below.

## Goal

Build a tool that shows a date and time from one zone in several other time zones.

## Core features

- Enter a base zone, date and time
- Add or remove cities (IANA zones)
- Show UTC offset and whether DST applies
- Highlight overlapping office hours (09–18)
- Copy the result table

## Inputs

- Base zone, date and time
- List of zones to compare

## Outputs

- Local date and time per zone
- Offset from the base zone

## Edge cases and handling rules

- Clearly mark date changes (+1 day, −1 day)
- Offset changes around DST start and end
- Half- and quarter-hour offsets (India, Nepal)
- Use IANA names rather than fixed offsets

## 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)

- 2026-01-15 09:00 Asia/Seoul → 2026-01-14 19:00 America/New_York
- 2026-07-15 09:00 Asia/Seoul → 2026-07-14 20:00 America/New_York
- Asia/Kolkata is +05:30

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-timezone-calculator
description: Use when building or changing a タイムゾーン計算機 tool. Build a tool that shows a date and time from one zone in several other time zones.
---

# Build: タイムゾーン計算機

## Goal

Build a tool that shows a date and time from one zone in several other time zones.

## 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

- Enter a base zone, date and time
- Add or remove cities (IANA zones)
- Show UTC offset and whether DST applies
- Highlight overlapping office hours (09–18)
- Copy the result table

## Inputs

- Base zone, date and time
- List of zones to compare

## Outputs

- Local date and time per zone
- Offset from the base zone

## Rules

- Clearly mark date changes (+1 day, −1 day)
- Offset changes around DST start and end
- Half- and quarter-hour offsets (India, Nepal)
- Use IANA names rather than fixed offsets
- 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

- [ ] 2026-01-15 09:00 Asia/Seoul → 2026-01-14 19:00 America/New_York
- [ ] 2026-07-15 09:00 Asia/Seoul → 2026-07-14 20:00 America/New_York
- [ ] Asia/Kolkata is +05:30