Fifteen seconds is a rest between lifts, a transition count, or the pause before everyone speaks at once. Count it on the screen, not in your head. Head-counts shrink when you are tired.
Start it on the exhale. If you start it while still talking, the fifteen belongs to the sentence, not the rest. You can ask for a notification, but only after you click. The browser will not prompt on load. If you allow it, the same moment the alarm plays, a notice says the timer is done. Mute the sound in the control row if you only want the notice.
This page is the 15 seconds countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 15 seconds timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Start it on the exhale. If you start it while still talking, the fifteen belongs to the sentence, not the rest.
What is 15 seconds actually good for?
Fifteen seconds is a rest between lifts, a transition count, or the pause before everyone speaks at once.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.