Shared countdown timer

Browser-based countdown timer with QR-code sharing. Open the app, set a duration, and share the synced countdown to any device via QR code or link.

Try the Shared countdown timer demo site.

How it works

All timer state is passed in the URL:

https://example.com/?s=1741718400&d=300

Param	Meaning
s	Start time — Unix epoch seconds (UTC)
d	Duration — seconds

A device scanning the QR code or opening the link reads these parameters and counts down from the same absolute start time. No server or websockets required. The countdown stays in sync because all devices reference the same UTC epoch.

UTC epoch

UTC epoch (Unix time) is the number of seconds since January 1, 1970 00:00:00 UTC. It is the same for every device on Earth regardless of time zone. When a timer is started, the current epoch second and the chosen duration are encoded into the URL. Any device that opens the link computes the time remaining as (start + duration) - now. Using UTC epoch ensures no server communication or real-time sync is required.

This works because modern devices keep their clocks accurate via NTP (Network Time Protocol), typically within a fraction of a second. As long as device clocks are reasonably accurate, all viewers see the same countdown within a second or two of each other.

Quickstart

npm install
npm run dev

Open the local URL, pick a duration, hit Start & Share, and scan the QR code from another device on the same network.

Build

npm run build    # outputs to dist/
npm run preview  # preview production build locally

Stack

• Vite — dev server and build tool
• Vanilla JS — no framework
• qrcode — QR code generation (npm)

Project Structure

index.html          Entry point
src/
  main.js           App orchestration — reads URL, wires UI
  timer.js          Timer math — pure functions (parse, remaining, format)
  qr.js             QR code rendering wrapper
  share.js          URL building and clipboard copy
  style.css         Styles