What a 30 seconds timer is for
Thirty seconds is a standard plank, stretch hold or interval set, and the length of a quick elevator pitch. The page loads with the countdown already set, so the whole interaction is one click.
The timer runs on the browser clock, not a server, so it works offline once the page is open and it keeps counting in a background tab. At zero it plays a short chime; keep the tab unmuted if you plan to walk away.