It depends entirely on how the timer is built. A timer that counts down by decrementing a number on every tick loses time the moment a browser throttles those ticks in a background tab. A timer that records an end time and checks the real clock does not, because it was never trusting the ticks to begin with. Our online timer uses the second method, verified against its own code, not just claimed.
What browsers actually do to background tabs
Browsers throttle timers in hidden tabs to save battery and CPU, and the mechanics are publicly
documented. Per MDN’s setTimeout reference,
Chrome checks standard background timers roughly once per second once a tab has been hidden, and
applies “intensive throttling,” checking only once per minute, once a page has been hidden for more
than 5 minutes (with a few other conditions). Chrome’s own engineering blog confirms the specifics:
“Heavy throttling of chained JS timers beginning in Chrome
88”, shipped January 2021. Firefox
desktop clamps inactive-tab timers to a 1-second minimum interval per MDN, and Firefox for Android
can throttle far more aggressively or unload the tab. iOS Safari goes further still, it can suspend
a backgrounded page’s JavaScript environment entirely rather than merely slowing it down.
None of this is a bug, it’s deliberate battery-saving behavior every major browser ships. The question that actually matters isn’t whether throttling happens, it’s whether the timer you’re using was built to survive it.
The two ways to build a countdown
Counting ticks. Start at your duration, and subtract a fixed amount every time a repeating timer fires. This is the simplest way to write a countdown, and the one every quick tutorial teaches. It’s also the one that drifts, if the browser only fires your tick once a minute instead of once a second, your countdown falls behind by however long it was throttled, then jumps to catch up once the tab becomes visible again.
Reading the clock. Record a target end time the moment the countdown starts (the current time plus your duration), and on every check, calculate the remaining time as end time minus the current time. This approach never depends on how often the tick actually fired, it recalculates
fresh from the real clock every time, so a throttled or delayed tick just means the next check
catches up instantly rather than compounding an error.
So does ours keep running?
Yes. The countdown records a target end time and computes remaining time against the system clock on every check, the same pattern described above, confirmed directly in the tool’s own code rather than assumed. Switching tabs, opening another window, or coming back after several minutes shows the correct remaining time, not a stale one that has to catch up. Try it yourself with the online timer, free, no signup.
The one thing that genuinely can fail: the sound
The visible countdown stays accurate. The alarm sound is a different story. Browser autoplay policy restricts audio in ways that vary by browser and by whether the tab has recent user interaction, and background audio on a phone is restricted more aggressively still. Being honest about the limit: if your phone screen locks, the page may be suspended entirely and the alarm may fire late or not play at all. For anything you genuinely must hear, keep the tab visible and the screen on rather than relying on a background alarm.
Phones, screen lock and sleep
On both iOS and Android, locking the screen or switching apps can suspend a web page’s JavaScript entirely rather than just throttling it, which is a stricter version of the same background-tab behavior described above. The practical effect: when you unlock and return, the on-screen number is still correct, because it’s calculated fresh from the end time and the current clock the instant the page resumes, but any alarm that was supposed to fire while the screen was locked may have been silently missed. The number catches up instantly; the sound does not.
How to test any timer in 60 seconds
A repeatable check you can run on any timer, not just ours: set it for 2 minutes, switch to another tab, wait 90 real seconds by a phone clock, then switch back. If the timer shows something close to 30 seconds remaining, it’s reading the clock correctly. If it shows noticeably more than 30 seconds remaining, it drifted while hidden, and you’ve just found a timer that will finish late when you need it most.
What to do when you need it to be reliable
Run it fullscreen on a second screen or a projector rather than in a background browser tab, so there’s no throttling question to begin with. Don’t minimize the browser or switch away if you’re relying on the alarm specifically. For measuring how long something actually took after the fact rather than counting down to an alert, the online stopwatch with lap times works the same way, elapsed time calculated against the real clock, not a running tally of ticks. Keep the device’s screen awake if you’re on a phone or tablet, since a locked screen is the one scenario that can suspend the page outright regardless of how the countdown itself is built.
Does it work offline?
Once the page has fully loaded, the countdown itself needs no server connection to keep running, it works entirely from your device’s own clock. Reloading the page, or navigating away and back, while offline is a different question, without built-in offline caching, the page itself won’t load without a connection. In practice: start your timer while you still have a connection, and you’re fine to lose it afterward, just don’t refresh the tab.
FAQ
Does an online timer keep running if I minimise the browser?
It depends entirely on how the timer is built. A timer that counts down by decrementing a number on every tick loses time once the browser throttles it. A timer that records an end time and checks the real clock does not, because it never trusted the ticks to begin with. Ours uses the second method.
Will the alarm sound if my phone is locked?
Not reliably. A locked screen can suspend the page entirely on iOS, and background audio is restricted by design on most mobile browsers. Keep the screen on and the tab visible for anything you genuinely must hear.
Why did my timer finish late?
Almost always because it was built by counting ticks rather than checking the clock, and the browser throttled those ticks while the tab was hidden. The countdown falls behind, then catches up abruptly once the tab is visible again.
Does a browser stopwatch drift?
The same rule applies. A stopwatch that adds up ticks drifts the same way a tick-based countdown does. One that measures elapsed time against the real clock does not.
Does an online timer work offline?
Once the page has fully loaded, the countdown itself needs no server, it runs entirely in your browser using the device clock. Reloading the page or navigating back to it while offline is a different matter, without offline caching built in, the page itself won’t load without a connection.
Trust the number, not the tick. Set up the online timer now, drift-corrected from the wall clock, free, no signup, jump straight to a 5 minute timer once that dedicated page is ready, and browse free browser tools for the rest of the toolkit.