D-Day 倒计时会将今天与目标日期比较，告诉你还剩多少个日历日，或已经过去多少天。
离考试还有几天？第 100 天是哪天？宝宝几个月大了？日期计算处处都可能差一天，所以要弄清楚用的是哪条规则。
D-day 计数法把目标日期记为 D，前一天记为 D-1，后一天记为 D+1。剩余天数就是目标日期减去今天。以 2026 年 10 月 28 日为准，12 月 25 日是 D-58。
开始：2026-03-01
经过天数：2026-06-08 是开始后的第 99 天
第几天： 2026-06-08 是第 100 天是否计算首日会如何改变倒计时和纪念日的结果、如何计算周岁年龄，以及按日历月计算的月龄和月末日期如何处理。阅读指南 →
想要自己的版本？把下面的任务提示词粘贴到 AI 聊天中，或将技能文件保存到 AI 编程助手。其中包含我们开发此工具时整理的功能、边界情况和测试用例。
提示词和技能以英文提供，AI 工具在任何语言环境下都能很好地理解。
直接粘贴到 Claude、ChatGPT、Gemini 等 AI 聊天中即可。
You are an experienced full-stack web developer. Build a web tool called "倒计时计算器" that meets the requirements below.
## Goal
Build a tool that shows the days remaining until, or elapsed since, a target date in D-day format.
## Core features
- Show D-n, D-Day or D+n for a target date
- Change the reference date
- 'Day number' mode that counts the first day
- Generate milestone lists such as day 100 or 1,000
- Copy the result sentence
## Inputs
- Target date
- Reference date (default today) and mode
## Outputs
- D-day label
- Days remaining or elapsed
- Milestone list
## Edge cases and handling rules
- Compare calendar dates only, not times
- Determine 'today' in the user's time zone
- Handle 29 February in leap years
- Show 'D-Day' when the target is today
## 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)
- From 2026-10-28, 2026-12-25 is D-58
- Day 100 from 2026-03-01 is 2026-06-08
- Same date shows D-Day
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.在 Claude Code 等 AI 编程助手中保存为 .claude/skills/build-d-day-calculator/SKILL.md，以后开发或修改此工具时可重复使用。
---
name: build-d-day-calculator
description: Use when building or changing a 倒计时计算器 tool. Build a tool that shows the days remaining until, or elapsed since, a target date in D-day format.
---
# Build: 倒计时计算器
## Goal
Build a tool that shows the days remaining until, or elapsed since, a target date in D-day format.
## 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
- Show D-n, D-Day or D+n for a target date
- Change the reference date
- 'Day number' mode that counts the first day
- Generate milestone lists such as day 100 or 1,000
- Copy the result sentence
## Inputs
- Target date
- Reference date (default today) and mode
## Outputs
- D-day label
- Days remaining or elapsed
- Milestone list
## Rules
- Compare calendar dates only, not times
- Determine 'today' in the user's time zone
- Handle 29 February in leap years
- Show 'D-Day' when the target is today
- 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
- [ ] From 2026-10-28, 2026-12-25 is D-58
- [ ] Day 100 from 2026-03-01 is 2026-06-08
- [ ] Same date shows D-Day