How this timer works

The interesting problem with a browser timer is not counting down — it is that the browser stops cooperating the moment you look away. Chrome checks timers in a hidden tab about once a second, and drops to once a minute once the tab has been hidden for five minutes. Firefox for Android clamps to a fifteen-minute minimum and may unload the page altogether. Any timer built by subtracting one second per tick loses exactly as much time as the browser withheld, and it loses it silently.

This one stores a single number when you press start: the instant it should finish, on your device's clock. Everything on screen is worked out by subtracting the current time from that instant. There is no counter to fall behind. Background the tab for ten minutes and the first frame after you return is already correct, because nothing was ever being accumulated.

Why the alarm is a separate problem from the display

Being correct and being punctual are different guarantees, and it is worth being honest about which one a web page can offer. The displayed time is always right. The alarm firing at the exact second is not something any browser tab can promise, because the same throttling that slows the display also delays the callback that plays the sound.

So there are three layers, and only the last one is a guarantee:

The display — recomputed every second while the tab is visible, and resynced the moment it becomes visible again.
A scheduled alarm — one long timeout set for the finish instant, plus the chime scheduled on the browser's audio clock, which runs on a separate thread from JavaScript timers. Better than nothing. Still delayable.
The catch-up — whenever the page becomes visible again, it checks whether the finish instant has passed and, if it has, says so immediately and tells you how long ago.This is the layer that cannot fail, because it does not depend on anything having run while you were away.

A common claim is that scheduling sound on the audio clock exempts a page from throttling. It does not. Chrome's exemption is for pages that have actually made a noise in the last thirty seconds, and a scheduled-but-not-yet-playing sound makes no noise during the countdown — which is precisely the window that matters. We schedule on the audio clock anyway, because a separate thread is genuinely more reliable than a JavaScript timer, but it buys robustness rather than a guarantee.

The sound is made, not downloaded

The alarm is three short rising tones generated by the browser's own audio hardware. Nothing is fetched, so there is no file to fail to load, no request to be blocked, and no delay on a slow connection. Browsers block audio until you have interacted with the page, which is why the sound is switched on by your first press of Start or Test sound — worth doing once before you rely on it.

Worked example: the ten-minute background tab

Set five minutes and switch tabs. Ten minutes later, come back. The timer shows finished and tells you it finished roughly five minutes ago, while the tab was in the background. What happened underneath: the scheduled alarm may or may not have fired on time, and the display certainly stopped updating — but the finish instant was fixed the moment you pressed Start, so working out what happened was a subtraction, not a recollection.

Worked example: a timer that outlives the page

Set twenty-five minutes, then close the tab entirely and reopen it half an hour later. You get a panel saying the timer finished five minutes ago, with the option to start it again. It does not play a sound — the browser would block that anyway, having had no interaction yet, and a sound for something that happened half an hour ago would be worse than useless.

Limitations, stated plainly

It is not a reliable alarm clock. If the alarm firing on time genuinely matters — a medication, a train — use a device alarm. A browser tab is the wrong tool and no amount of engineering changes that.
One timer at a time. Running several at once is planned but not built; the page will say so plainly rather than implying otherwise.
Notifications are not offered. The API throws on most mobile browsers and making it work would require a service worker, which would stop this being a purely static site. The visual finish state and the tab title are the signals that always work.
A device clock change moves the finish time with it, because the finish time is an instant on that clock.
Saved in this browser only. Clearing site data or switching devices will erase it.

Sources

Chrome: timer throttling in background tabs — the once-a-second and once-a-minute figures, and the conditions for the audio exemption.
MDN: setTimeout — Firefox desktop and Firefox for Android clamping.
MDN: autoplay guide — why sound needs a gesture before it will play.
WCAG 2.2.1 Timing Adjustable — why a timer you set yourself is not a time limit set by the content.

Questions

Will the alarm go off if I switch tabs?

Usually, but possibly late — up to about a minute on desktop and longer on Android, because browsers deliberately slow background tabs down. The timer itself stays exact, and when you come back the page tells you precisely when it finished. If the alarm firing at the right second matters, keep the tab visible or use a device alarm.

What happens if I close the tab or reload?

The timer keeps its finish time, so a reload picks up exactly where it was. If you close the tab and come back after it should have finished, you get a panel telling you when it finished rather than silence.

Why is there no sound the first time?

Browsers block audio until you have interacted with the page. Your first press of Start switches it on. Test sound does the same thing and lets you check the volume before you rely on it.

Can I run more than one timer?

Not yet — this page runs one timer at a time. If you need to track two things at once, thestopwatch records laps against a single running clock, which covers some of the same ground.

Is it accurate?

The countdown is as accurate as your device's clock, and it does not drift regardless of what the browser does to the tab. What is not guaranteed is the alarm's punctuality when the page is hidden — those are different claims and this page tries to be clear about which is which.

Does it work with a screen reader?

The counting figure is deliberately silent, because a readout that announced itself every second would be unusable. Instead you get announcements at proportional milestones — halfway, five minutes, one minute, thirty seconds, depending on how long the timer is — plus start, pause and reset, and a single alert when it finishes.

Can I use the keyboard?

Yes. Space starts and pauses and R resets, as long as focus is on the tool and not in one of the number fields. Every control is also a normal button reachable withTab.

Timer