Online Stopwatch

How to use Online Stopwatch

1. Press Start to begin.
2. Press Lap at each checkpoint — times are listed newest first.
3. Pause to stop temporarily; pressing Start again continues from where it stopped.
4. Reset clears both the clock and the lap list.
5. Pause preserves the elapsed total — only Reset clears it.
6. Works offline once the page has loaded.
7. Closing the tab ends the run; switching to another tab does not.

It uses a monotonic clock, not the wall clock

Timing is taken from performance.now(), a counter that only ever moves forward, rather than the system clock. That means an NTP correction, a daylight-saving change, a timezone switch or someone adjusting the device clock mid-run cannot corrupt the measurement — all of which can and do happen to a naive timer built on the current date. For a run measured in hours, this is the difference between a reliable result and a mysterious one.

What happens when you switch tabs

The display is driven by the browser’s animation loop, which is throttled or suspended entirely in a background tab to save battery, so the digits may freeze while you are elsewhere. The measurement does not. Elapsed time is recalculated from the start timestamp on every frame rather than accumulated tick by tick, so when you come back the reading jumps straight to the correct value. You can safely leave it running in another tab. Phones are less forgiving: locking the screen may suspend the page altogether, so keep the display awake for a long timed run.

Laps are totals, not splits

Pressing Lap records the total elapsed time at that instant, newest at the top. To get the duration of an individual lap, subtract the row below it — so laps at 1:02.40 and 2:08.15 mean a second lap of 1:05.75. Recording cumulative times is the convention in athletics because it avoids the rounding drift you get from adding up individually rounded splits.

How precise it really is

The readout shows hundredths. Browsers deliberately coarsen their high-resolution timers — usually to somewhere between 100 microseconds and a millisecond — as a defence against timing side-channel attacks, which is still far finer than the display. The genuine limit is you: human reaction time on a start or stop press is around two tenths of a second, so treat the hundredths as tidy rather than authoritative for anything hand-triggered.