Unixタイムスタンプ変換器

You are an experienced full-stack web developer. Build a web tool called "Unixタイムスタンプ変換器" that meets the requirements below.

## Goal

Build a tool that converts between Unix timestamps and human-readable dates.

## Core features

- Timestamp → date and date → timestamp
- Auto-detect seconds (10 digits) or milliseconds (13 digits)
- Show UTC, local and a chosen IANA zone together
- ISO 8601 output
- 'Now' button

## Inputs

- A timestamp or a date and time
- Time zone

## Outputs

- Converted date or timestamp
- ISO 8601 string
- Relative time (e.g. 3 days ago)

## Edge cases and handling rules

- Support negative timestamps before 1970
- Handle values after 2038 correctly
- Explain skipped or repeated local times at DST transitions
- Validate non-numeric input

## 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)

- 0 → 1970-01-01T00:00:00Z
- 1700000000 → 2023-11-14T22:13:20Z
- 1700000000000 (13 digits) is the same instant

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-timestamp-converter
description: Use when building or changing a Unixタイムスタンプ変換器 tool. Build a tool that converts between Unix timestamps and human-readable dates.
---

# Build: Unixタイムスタンプ変換器

## Goal

Build a tool that converts between Unix timestamps and human-readable dates.

## 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

- Timestamp → date and date → timestamp
- Auto-detect seconds (10 digits) or milliseconds (13 digits)
- Show UTC, local and a chosen IANA zone together
- ISO 8601 output
- 'Now' button

## Inputs

- A timestamp or a date and time
- Time zone

## Outputs

- Converted date or timestamp
- ISO 8601 string
- Relative time (e.g. 3 days ago)

## Rules

- Support negative timestamps before 1970
- Handle values after 2038 correctly
- Explain skipped or repeated local times at DST transitions
- Validate non-numeric input
- 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

- [ ] 0 → 1970-01-01T00:00:00Z
- [ ] 1700000000 → 2023-11-14T22:13:20Z
- [ ] 1700000000000 (13 digits) is the same instant