Timers that work in a background tab (and why some do not)
The throttling problem
Browsers save power by throttling JavaScript in background tabs, and each browser draws the line differently. Animation frames stop almost immediately, so any timer built on a visual loop drifts or freezes. Timers built on wall-clock time, where the countdown is computed from the actual current time rather than counted seconds, keep accurate time even when the tab is throttled, because the browser still wakes them periodically to check.
What survives in the background
SetInterval-based timers get throttled to roughly once per second or worse in background tabs, but they do still fire, which is why a well-built timer keeps running and only the smooth animation of the digits suffers. Audio alerts are the other risk: some browsers block sound in unvisited background tabs unless the tab has had user interaction. A timer page you clicked to start has interacted, so its alarm will play; a timer that starts itself will not be so lucky.
Notifications as backup
The most robust browser timers fire both an in-page alarm and a system notification, so the alert reaches you even if the browser window is minimized or you are in another application. Permission for notifications is requested on first use. For long countdowns (an hour or more, like a meeting or laundry cycle) this matters more than precision; for short intervals (cooking, exercise rounds) the audible alarm does the work and the notification is a convenience.
Why phones are a mixed bag
Mobile browsers throttle background tabs harder than desktop ones, and some lock the screen entirely, which can pause or kill a web timer. A web timer works well on a phone when the screen stays on and the tab is visible; it is the wrong tool for timing a phone in your pocket. For pocket scenarios, a native app or a physical timer wins. Know which scenario you are in before you rely on any countdown.
Testing your timer once
Set any new timer to 1 minute, switch tabs, and wait. If the alarm rings within a few seconds of the minute, the timer survives throttling and you can trust it. If it rings late or not at all, you have learned that in two minutes what you would otherwise learn while your pasta boiled over. Our timer passes this test on desktop browsers and alerts from a background tab, which is precisely why we built it.
Tools
Our free online timer is built on wall-clock time and rings audibly from a background tab on desktop. Test it once with the 1-minute trick above, keep the volume up, and you have a countdown that behaves like an appliance, not a webpage.