Stopwatch

How the stopwatch works

Everything begins with a timestamp. When you press Start, the page reads a monotonic high-resolution clock and remembers that instant as the run's origin. Every animation frame it reads the clock again, subtracts the origin, subtracts any time that had already been banked from previous runs, and writes the result into the readout. Nothing is ever incremented, so a slow frame, a throttled background tab or a sleeping laptop cannot make the total drift — the stopwatch simply reports the difference between two clock readings whenever it is asked.

Lap and split times come from the same idea. Pressing Lap records the current total as a new split. The lap time shown next to it is that split minus the previous one, and the final lap is always the latest split minus the previous split. Because both figures are derived from stored timestamps rather than from arithmetic done at press time, the table stays internally consistent: the splits always add up to the total on the readout.

Stop is a real pause, not a reset: the total freezes, the laps stay in the table, and pressing Start again continues from the frozen total as though no time had passed in between. That makes the tool usable for anything that gets interrupted — a set of stretches, a queue of tasks or an interval workout — without losing the earlier laps.

Accuracy and honest limits

The clock source is the browser's high-resolution monotonic timer, which is deliberately independent of the system date, so changing the computer's clock or crossing a daylight-saving boundary cannot corrupt a measurement. What limits the numbers you actually see is painting, not measuring.

• Display resolution follows the frame rate. The millisecond field updates each time the browser paints, roughly every 16 ms on a 60 Hz screen. The underlying elapsed value is finer than that, but no on-screen digit can update faster than the screen does, so treat the last digit as indicative rather than authoritative.
• Hidden tabs may not repaint. The stopwatch keeps correct time in a background tab, but the digits can appear frozen when you return for a fraction of a second before the next frame catches up. If you need to read the numbers continuously, keep the tab visible.
• Laps are manual. A lap is recorded at the moment you press the button, so your own reaction time — typically a couple of tenths of a second for a mouse or keyboard press — is part of the measurement. For anything where precision matters, press and hold nothing extra: one clean press per lap gives the most repeatable result.

How to use it

1. Start timing. Press Start or the Space key. The readout shows minutes, seconds and milliseconds, and grows an hours segment automatically once the run passes an hour.
2. Record laps. Press Lap or L at each marker. A row appears with the lap number, that lap's own duration and the running split total.
3. Read the markers. As soon as two laps exist, the fastest is badged green and the slowest red, both in the table and in the summary tiles above it. Ties are left unmarked rather than picked arbitrarily.
4. Stop and resume. Press Stop or Space to freeze the total; pressing Start again continues from the same total, so an interruption costs nothing.
5. Reset for a clean run. Press Reset or R to zero the readout and empty the laps table — export first if you want to keep them.
6. Export. Export laps as CSV builds a file named for the date with columns for lap, lap time, split and marker, and downloads it straight to your device.

Every shortcut is suppressed while the focus sits in a text field, so typing a note in another part of the page can never trigger a lap or a reset by accident. All timing happens locally: no data is sent anywhere, no account is needed, and the page works with the network switched off.