Age in Months Card Generator

Calculate age in months and create a cute downloadable card saying how many months old someone is.

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// about

About this Tool

Enter a name and birth date to create a playful card showing the calendar-month age. It is inspired by the cute phrase parents use for babies, even when the number gets wonderfully large.

How to use it

  1. Enter the person's name and birth date.
  2. Choose the reference date and click the months button.
  3. Download the cute card or copy the sentence to share.

Why people use this tool

  • A fun way to share birthdays and month milestones with family and friends.
  • Creates a ready-to-download image and a copyable sentence.
  • Uses the reference date shown on the card so the result is easy to understand.
Guides · Date & time · 7 min read

Date Calculations Without the Off-by-One: D-Days, Anniversaries, Age and Months Old

How many days until the exam? When is the 100th day? How many months old is the baby? Date maths is full of one-day differences, so it pays to know the rule being used.

D-day counting labels the target date D, the day before D-1 and the day after D+1. The days remaining are simply target date minus today. On 28 October 2026, 25 December is D-58.

Start: 2026-03-01
Elapsed days: 2026-06-08 is 99 days after the start
Day number:   2026-06-08 is the 100th day
How counting the first day changes D-day and anniversary results, how to calculate age in years, and how calendar-month ages and month-end dates work.
Read guide →
Build it with AI

Build this tool yourself with AI

Want your own version of this tool? Copy the work-order prompt into an AI chat, or save the skill file for an AI coding agent. Both describe the features, edge cases and test cases we used when building it.

AI work-order prompt

Paste it as-is into an AI chat such as Claude, ChatGPT or Gemini.

You are an experienced full-stack web developer. Build a web tool called "Age in Months Card Generator" that meets the requirements below.

## Goal

Build a tool that calculates age in calendar months from a name and birth date and creates a shareable card image.

## Core features

- Calendar-month age
- Choose the reference date (default today)
- Generate a sentence such as 'Alex is 18 months old'
- Download the card as SVG/PNG
- Copy the sentence

## Inputs

- Name
- Birth date
- Reference date

## Outputs

- Age in months
- Share sentence
- Card image

## Edge cases and handling rules

- Subtract one month if the reference day is before the birth day
- Error when the reference date is before the birth date
- Document the rule for birthdays on the 31st
- Keep the card layout intact with long names

## 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)

- Born 1975-10-22, reference 2026-08-12 → 609 months
- Born 15 May: 0 months on 14 June, 1 month on 15 June
- The downloaded image contains the name and months

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.

Skill (SKILL.md)

Save it as .claude/skills/build-baby-age-calculator/SKILL.md for AI coding agents such as Claude Code to reuse whenever you build or change this tool.

---
name: build-baby-age-calculator
description: Use when building or changing a Age in Months Card Generator tool. Build a tool that calculates age in calendar months from a name and birth date and creates a shareable card image.
---

# Build: Age in Months Card Generator

## Goal

Build a tool that calculates age in calendar months from a name and birth date and creates a shareable card image.

## 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

- Calendar-month age
- Choose the reference date (default today)
- Generate a sentence such as 'Alex is 18 months old'
- Download the card as SVG/PNG
- Copy the sentence

## Inputs

- Name
- Birth date
- Reference date

## Outputs

- Age in months
- Share sentence
- Card image

## Rules

- Subtract one month if the reference day is before the birth day
- Error when the reference date is before the birth date
- Document the rule for birthdays on the 31st
- Keep the card layout intact with long names
- 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

- [ ] Born 1975-10-22, reference 2026-08-12 → 609 months
- [ ] Born 15 May: 0 months on 14 June, 1 month on 15 June
- [ ] The downloaded image contains the name and months

Tips for working with AI

  • Tell the AI your stack (for example React, Vue or plain HTML) on the first line so the code matches your project.
  • Ask it to run the listed test cases and show the results before you accept the code.
  • Never paste real secrets, customer data or API keys into the prompt; use sample values instead.