What a 30 seconds timer is good for
Plank holds and wall sits, rest between sets, a breath-hold or box-breathing round, brushing one quadrant of teeth, turn-taking for kids, a quick microwave check, or the pause before a speech.
The countdown runs against your device clock, so it stays accurate if the tab is throttled in the background. At zero it fills the screen with a Stop button, plays your chosen sound, and vibrates on phones. Need several at once? The multi-timer runs them side by side.