Measure an elapsed interval and record checkpoints while it runs. This dedicated page renders the Stopwatch only. For an alarm or countdown alongside it, stay in the all-in-one workspace.
Start, stop and resume
Start begins from the displayed elapsed value. Stop freezes that value; Start continues from it. Reset stops the run, returns the display to zero and clears the lap list. Decide whether to keep a record before resetting: XClock has no lap export or undo control.
What a lap means here
Lap records the total elapsed reading since the run began, without stopping it. If the list reads 00:00:12.30 and then 00:00:29.10, the second value is a cumulative checkpoint. The segment between those checkpoints is 16.80 seconds; the list is not claiming that both segments lasted 29.10 seconds. You must calculate segment differences yourself.
For a presentation rehearsal, press Lap at the end of the introduction, explanation and conclusion. Stop at the end. Compare the checkpoints with your own notes. For repeated short actions, reset between runs if you want each reading to begin at zero rather than accumulating laps.
Hundredths are display resolution, not a guarantee
The display formats elapsed milliseconds into hours, minutes, seconds and hundredths. While running, the implementation computes elapsed time from the device’s clock; it does not add exactly 10 milliseconds on every screen update. A delayed repaint can therefore skip visible readings. The number of digits does not establish measurement accuracy.
This implementation uses the device’s wall clock, not a laboratory timebase. Manual clock changes or time synchronization can affect a run. Human reaction time also affects the start, lap and stop points. Do not use the result for official competition, safety-critical measurement or scientific calibration. See how XClock’s timing methods differ.
Background tabs and page closure are different
If the page remains running in a background tab, its display callbacks may be delayed; when a callback runs, the stopwatch calculates against the device clock. That is different from keeping every hundredth visibly updated. We do not guarantee operation through device sleep, tab suspension or process termination.
Before leaving, the page attempts to save the current elapsed reading, running flag and laps. When reopened, a running stopwatch continues from its saved reading. It does not include the time the page was closed. Browser termination may also prevent the latest save. Stop the watch before leaving if you need to review a stable reading later.
Keep a record you can use
Write down the elapsed result and checkpoint labels before clearing site data or switching devices. Browser storage is local; it is not a backed-up XClock account. The storage guide explains restoration and separate browser profiles.
For an alert at the end of an interval, choose the countdown timer. A stopwatch does not ring automatically. See testing and known limitations for the scope of the actual checks.