Timer & Stopwatch
Stopwatch, countdown and laps
A stopwatch with laps and a countdown timer, both measured from the clock rather than by counting ticks, so neither drifts.
Timer & Stopwatch
Runs in your browser · nothing uploaded01 — How to use Timer & Stopwatch
02 — Getting good results
03 — Questions
Only to load the page. Everything after that uses your device’s own clock, and nothing is transmitted.
Yes. The browser may stop updating the display to save power, but the elapsed time is measured from timestamps rather than counted, so the reading is correct as soon as the tab is visible again.
No. It shows that time is up and stops the clock. Browsers block sound from pages the user has not interacted with, and a timer that sometimes makes a noise and sometimes does not is worse than one that reliably does not.
It uses a monotonic high-resolution clock, which does not jump if the system time changes. Display resolution is ten milliseconds. Browsers deliberately reduce timer precision slightly as a defence against timing attacks, so treat it as accurate to a few milliseconds rather than to the microsecond.
04 — More about Timer & Stopwatch
Why counting ticks is the wrong way to measure time
A timer callback scheduled for every ten milliseconds does not fire every ten milliseconds. It fires no sooner than that, and often considerably later: the browser must finish whatever it is doing first, and under load or on a busy page the delay compounds. Worse, every major browser now throttles timers in background tabs to at most once per second, and sometimes suspends them entirely, specifically to stop idle pages draining batteries. Code that adds a fixed increment per callback therefore under-counts, and the error is unbounded — it grows with how long the timer runs and how busy the machine is. The fix is to stop counting and start measuring: record the timestamp at which the clock started, and compute the difference whenever you need to display something. The callback then only controls how often the screen updates, which is a presentation concern rather than a correctness one.
Monotonic time
There are two clocks available to a page. One reports the current date and time, and it can move backwards: the system may correct itself against a network time source, the user may change the time zone, daylight saving may shift. Measuring a duration by subtracting two readings from that clock can produce a negative interval or a jump of an hour. The other is a monotonic clock that only ever moves forward, measuring from an arbitrary origin, and it exists precisely for measuring elapsed time. This tool uses the monotonic one, which is why an adjustment to your system clock mid-run has no effect on a running stopwatch.
Laps and splits
A lap time and a split time are different measurements and are frequently confused. The lap is the duration of one segment — the time since the previous lap was taken. The split is the total elapsed at that point. Both are useful and they answer different questions: the lap tells you how that segment compared to the others, the split tells you where you are against an overall target. This tool shows the lap prominently and the split alongside it, and marks the fastest and slowest segments once there are at least two to compare, since highlighting a single lap as both best and worst conveys nothing.
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where this came from.
Same shell, same shortcuts, same promise: nothing leaves your browser.