DEV3 Free Age and Date Difference Calculator — Source Code (React + Tailwind)

Source code of the DEV3 Age and Date Difference Calculator, an age, date difference and add/subtract date calculator with business-day counting and a holiday list, computed in UTC to avoid daylight-saving drift.

Free download: sign in to any DEV3 account and click Download ZIP. Try it live: https://dev3.tech/tools/age-date-difference-calculator shows the finished tool running.

What is in the download

• A standalone Vite + React + TypeScript + Tailwind CSS project: src/tool holds the tool, shared pieces sit beside it
• package.json with exact versions plus a lockfile, so npm ci works
• README.md (run, build, embed in Next.js or plain React) and an MIT LICENSE

What the code does

• Exact years, months and days with month-end clamping
• Business days between dates and when adding days, with custom holidays
• Inclusive/exclusive end-date counting
• Next-birthday countdown including leap-day birthdays

Tech stack

React 19, TypeScript, Vite, Tailwind CSS 4.

Install and customise

The How to install and How to use tabs list the steps. In short: unzip, npm ci, npm run dev, then copy the tool folder into your own app if you want to embed it.

Good to know

• A browser-only tool: there is no backend or database in the code.
• Weekends are fixed to Saturday and Sunday; regional work weeks are not modelled.