Two timers, one page
A stopwatch counts up from zero; a countdown counts down to it. Both are on this page because people rarely need only one, and switching between tabs to find the other is a small but constant irritation.
Why timing accuracy is not trivial
The obvious way to build a timer is to increment a counter every interval. This drifts badly, because browsers do not guarantee interval precision and deliberately throttle timers in background tabs to save battery — in some cases to once per minute.
Both timers here instead record a starting timestamp and calculate elapsed time from the system clock on each update. The display refresh may stutter if the tab is throttled, but the underlying time stays correct. Leave the countdown running in a background tab and it will still finish at the right moment.
Lap times
The stopwatch records both figures that matter when timing repeated efforts. The lap time is the duration of that individual segment, and the cumulative time is the total elapsed since starting.
Both are shown because they answer different questions. Lap times reveal consistency and pacing — whether your third interval was slower than your first. Cumulative time tells you where you stand against a target overall. Runners, swimmers and anyone timing repeated sets need both.
The countdown alert
The countdown sounds an audible alert and updates the browser tab title, so you can work in another tab and still notice when time expires. The tab title also shows the remaining time while running, which is useful when the timer is not the page you are looking at.
Note that browsers require some user interaction on a page before allowing audio playback. Since you have to press start, this is satisfied in normal use.
Common uses
Timing workouts, intervals and rest periods. Keeping presentations and meeting slots to time. Cooking, where a countdown is genuinely safety-relevant. Timed exams and practice tests. Tracking how long tasks actually take, which is usually longer than estimated. Board games with turn limits.
Precision expectations
The stopwatch displays hundredths of a second, which is appropriate for most purposes. Bear in mind that human reaction time when pressing start and stop is typically around 200 milliseconds, so for anything competitive the limiting factor is you rather than the timer.
Runs offline in your browser
Both timers work entirely locally. Nothing is transmitted or stored, and they continue working with no connection once the page has loaded.
Frequently Asked Questions
Does the timer keep running if I switch tabs?
Yes. Both timers calculate from timestamps rather than counting intervals, so background throttling affects only the display refresh, not the actual time.
What is the difference between lap time and total time?
Lap time is the duration of that individual segment; total time is the cumulative elapsed time since starting. Both are shown for each lap.
Why did the alert not sound?
Browsers block audio until a page has been interacted with. Pressing start satisfies this, so it should sound in normal use.
How accurate is the stopwatch?
It displays hundredths of a second and derives time from the system clock. Human reaction time of around 200 milliseconds is the practical limit for manual timing.
Does it work offline?
Yes. Once the page has loaded, both timers work with no connection.