How it is calculated
The countdown runs in the browser on its own clock, and nothing is sent to a server. Pause keeps the time left, and reset brings back the chosen period. The sound is a short beep that repeats until you press Stop.
Time left = chosen period − elapsed time
Example
The 5 min button sets 05:00. Start, pause at 03:12, then start again. At zero, a sound and the label "Time's up".
Questions
Why is there no sound before Start?
The browser allows sound only after a tap. The first Start unlocks the beep, so it can play when the countdown ends.
Can I set a time that is not in the list?
Yes. Minutes and seconds next to the buttons set any other period up to 9 hours 59 minutes. While the timer is running, the period stays put. On pause, a new button starts a different period from the beginning.
Does the phone screen turn off?
While the countdown runs, the page asks the screen to stay on. If the system refuses, the timer still reaches zero in the open tab.